[Sony Xperia P, U, Sola, Go] swissly:
munjeni, good obvervation, the device I am trying to unbrick is indeed not a z1c but a Xperia Go ST27i. So here is the right place to try to unbrick it and not there: https://xdaforums.com/sony-xperia-z1-compact/development/rom-t3657384/page115

So the device is an Xperia Go ST27i and the firmware I am using is "ST27i_6.2.A.1.100_1264_7472.ftf"

Today I tried a different driver for flashing, one I found in an S1tool package. Now the device name is no longer "S1Boot Download" but "SEMC Flash Device". Now there seems to work more but still it is not flashing correctly. The results in flashtool:

25/031/2018 19:31:10 - DEBUG - Creating bundle from ftf file : H:\ST27i_6.2.A.1.100_1264_7472.ftf
25/031/2018 19:31:10 - DEBUG - Added this entry to the bundle list : loader.sin
25/031/2018 19:31:10 - DEBUG - Added this entry to the bundle list : kernel.sin
25/031/2018 19:31:10 - DEBUG - Added this entry to the bundle list : system.sin
25/031/2018 19:31:10 - DEBUG - Added this entry to the bundle list : fotakernel.sin
25/031/2018 19:31:10 - DEBUG - Added this entry to the bundle list : modemfs.sin
25/031/2018 19:31:10 - DEBUG - Added this entry to the bundle list : partition-image.sin
25/031/2018 19:31:10 - DEBUG - Added this entry to the bundle list : simlock.ta
25/031/2018 19:31:10 - DEBUG - Added this entry to the bundle list : prcmu.sin
25/031/2018 19:31:10 - DEBUG - Added this entry to the bundle list : apps_log.sin
25/031/2018 19:31:10 - DEBUG - Added this entry to the bundle list : modem.sin
25/031/2018 19:31:10 - DEBUG - Added this entry to the bundle list : userdata.sin
25/031/2018 19:31:10 - DEBUG - Added this entry to the bundle list : cache.sin
25/031/2018 19:31:17 - INFO - Selected Bundle for Sony Xperia GO(ST27i). FW release : 6.2.A.1.100. Customization : 1264-7472
25/031/2018 19:31:17 - INFO - Preparing files for flashing
25/031/2018 19:31:17 - DEBUG - Created the prepared folder
25/031/2018 19:31:17 - DEBUG - Saving entry cache.sin to disk
25/031/2018 19:31:17 - DEBUG - Streaming from jar entry : cache.sin
25/031/2018 19:31:18 - DEBUG - Writing Entry to C:\Users\Bernhard\.flashTool\firmwares\prepared\cache.sin
25/031/2018 19:31:18 - DEBUG - Saving entry fotakernel.sin to disk
25/031/2018 19:31:18 - DEBUG - Streaming from jar entry : fotakernel.sin
25/031/2018 19:31:18 - DEBUG - Writing Entry to C:\Users\Bernhard\.flashTool\firmwares\prepared\fotakernel.sin
25/031/2018 19:31:18 - DEBUG - Saving entry kernel.sin to disk
25/031/2018 19:31:18 - DEBUG - Streaming from jar entry : kernel.sin
25/031/2018 19:31:18 - DEBUG - Writing Entry to C:\Users\Bernhard\.flashTool\firmwares\prepared\kernel.sin
25/031/2018 19:31:18 - DEBUG - Saving entry modem.sin to disk
25/031/2018 19:31:18 - DEBUG - Streaming from jar entry : modem.sin
25/031/2018 19:31:18 - DEBUG - Writing Entry to C:\Users\Bernhard\.flashTool\firmwares\prepared\modem.sin
25/031/2018 19:31:18 - DEBUG - Saving entry modemfs.sin to disk
25/031/2018 19:31:18 - DEBUG - Streaming from jar entry : modemfs.sin
25/031/2018 19:31:18 - DEBUG - Writing Entry to C:\Users\Bernhard\.flashTool\firmwares\prepared\modemfs.sin
25/031/2018 19:31:18 - DEBUG - Saving entry partition-image.sin to disk
25/031/2018 19:31:18 - DEBUG - Streaming from jar entry : partition-image.sin
25/031/2018 19:31:18 - DEBUG - Writing Entry to C:\Users\Bernhard\.flashTool\firmwares\prepared\partition-image.sin
25/031/2018 19:31:18 - DEBUG - Saving entry prcmu.sin to disk
25/031/2018 19:31:18 - DEBUG - Streaming from jar entry : prcmu.sin
25/031/2018 19:31:18 - DEBUG - Writing Entry to C:\Users\Bernhard\.flashTool\firmwares\prepared\prcmu.sin
25/031/2018 19:31:18 - DEBUG - Saving entry system.sin to disk
25/031/2018 19:31:18 - DEBUG - Streaming from jar entry : system.sin
25/031/2018 19:31:18 - DEBUG - Writing Entry to C:\Users\Bernhard\.flashTool\firmwares\prepared\system.sin
25/031/2018 19:31:30 - DEBUG - Saving entry loader.sin to disk
25/031/2018 19:31:30 - DEBUG - Streaming from jar entry : loader.sin
25/031/2018 19:31:30 - DEBUG - Writing Entry to C:\Users\Bernhard\.flashTool\firmwares\prepared\loader.sin
25/031/2018 19:31:30 - INFO - Please connect your device into flashmode.
25/036/2018 19:36:04 - INFO - Device connected in flash mode
25/036/2018 19:36:04 - INFO - Opening device for R/W
25/036/2018 19:36:04 - INFO - Reading device information
25/036/2018 19:36:04 - DEBUG - Reading packet from phone
25/036/2018 19:36:04 - DEBUG - IN : CommandID : 1 / Flags : false,false,false / Data length : 82 / Data CRC32 : 25 19 83 58
25/036/2018 19:36:04 - DEBUG - PROT_VER="04";ID="1240-4128";VER="R10E013";TYPE="S1_BOOT";ACTIVE_COMP_AID="0001";
25/036/2018 19:36:04 - DEBUG - First command reply (hook) :
25/036/2018 19:36:04 - INFO - Phone ready for flashmode operations.
25/036/2018 19:36:04 - INFO - Opening TA partition 2
25/036/2018 19:36:05 - DEBUG - Writing packet to phone
25/036/2018 19:36:05 - DEBUG - OUT : CommandID : 9 / Flags : false,true,false / Data length : 1 / Data CRC32 : EE 0E 61 2C
25/036/2018 19:36:05 - DEBUG - Reading packet from phone
25/036/2018 19:36:05 - DEBUG - IN : CommandID : 9 / Flags : false,false,false / Data length : 0 / Data CRC32 : 00 00 00 00
25/036/2018 19:36:05 - DEBUG - Writing packet to phone
25/036/2018 19:36:05 - DEBUG - OUT : CommandID : 12 / Flags : false,true,false / Data length : 4 / Data CRC32 : F0 01 48 CC
25/036/2018 19:36:05 - DEBUG - Reading packet from phone
25/036/2018 19:36:05 - DEBUG - IN : CommandID : 12 / Flags : false,false,false / Data length : 29 / Data CRC32 : F4 B2 2F 68
25/036/2018 19:36:05 - DEBUG - Writing packet to phone
25/036/2018 19:36:05 - DEBUG - OUT : CommandID : 12 / Flags : false,true,false / Data length : 4 / Data CRC32 : 5D EC A5 43
25/036/2018 19:36:05 - DEBUG - Reading packet from phone
25/036/2018 19:36:05 - DEBUG - IN : CommandID : 12 / Flags : false,false,false / Data length : 10 / Data CRC32 : 50 3C 3A 62
25/036/2018 19:36:05 - DEBUG - Writing packet to phone
25/036/2018 19:36:05 - DEBUG - OUT : CommandID : 12 / Flags : false,true,false / Data length : 4 / Data CRC32 : 46 67 65 F1
25/036/2018 19:36:05 - DEBUG - Reading packet from phone
25/036/2018 19:36:05 - DEBUG - IN : CommandID : 12 / Flags : false,false,false / Data length : 29 / Data CRC32 : F4 B2 2F 68
25/036/2018 19:36:05 - DEBUG - Writing packet to phone
25/036/2018 19:36:05 - DEBUG - OUT : CommandID : 12 / Flags : false,true,false / Data length : 4 / Data CRC32 : D8 03 F0 52
25/036/2018 19:36:05 - DEBUG - Reading packet from phone
25/036/2018 19:36:05 - DEBUG - IN : CommandID : 12 / Flags : false,false,false / Data length : 29 / Data CRC32 : F4 B2 2F 68
25/036/2018 19:36:05 - DEBUG - Writing packet to phone
25/036/2018 19:36:05 - DEBUG - OUT : CommandID : 12 / Flags : false,true,false / Data length : 4 / Data CRC32 : DF 6E 34 4B
25/036/2018 19:36:05 - DEBUG - Reading packet from phone
25/036/2018 19:36:05 - DEBUG - IN : CommandID : 12 / Flags : false,false,false / Data length : 29 / Data CRC32 : F4 B2 2F 68
25/036/2018 19:36:05 - INFO - Current device : Unknown: Mar 28 2013/20:01:29 - YT9105QUMF - Unknown: Mar 28 2013/20:01:29 - Unknown: Mar 28 2013/20:01:29 - Unknown: Mar 28 2013/20:01:29
25/036/2018 19:36:05 - INFO - Closing TA partition
25/036/2018 19:36:06 - DEBUG - Writing packet to phone
25/036/2018 19:36:06 - DEBUG - OUT : CommandID : 10 / Flags : false,true,false / Data length : 0 / Data CRC32 : 00 00 00 00
25/036/2018 19:36:06 - DEBUG - Reading packet from phone
25/036/2018 19:36:06 - DEBUG - IN : CommandID : 10 / Flags : false,false,false / Data length : 0 / Data CRC32 : 00 00 00 00
25/036/2018 19:36:06 - INFO - Start Flashing
25/036/2018 19:36:06 - INFO - Processing loader.sin
25/036/2018 19:36:06 - INFO - Checking header
25/036/2018 19:36:06 - DEBUG - Writing packet to phone
25/036/2018 19:36:06 - DEBUG - OUT : CommandID : 5 / Flags : false,true,false / Data length : 1020 / Data CRC32 : 97 FE FC 54
25/036/2018 19:36:06 - DEBUG - Reading packet from phone
25/036/2018 19:36:06 - DEBUG - IN : CommandID : 5 / Flags : false,false,false / Data length : 0 / Data CRC32 : 00 00 00 00
25/036/2018 19:36:06 - INFO - Flashing data
25/036/2018 19:36:06 - DEBUG - Number of parts to send : 2 / Part size : 65536
25/036/2018 19:36:06 - DEBUG - Sending part 1 of 2
25/036/2018 19:36:06 - DEBUG - Writing packet to phone
25/036/2018 19:36:06 - DEBUG - OUT : CommandID : 6 / Flags : false,true,true / Data length : 65536 / Data CRC32 : CB 68 A2 19
25/036/2018 19:36:06 - DEBUG - Reading packet from phone
25/036/2018 19:36:06 - DEBUG - IN : CommandID : 6 / Flags : false,false,false / Data length : 0 / Data CRC32 : 00 00 00 00
25/036/2018 19:36:06 - DEBUG - Sending part 2 of 2
25/036/2018 19:36:06 - DEBUG - Writing packet to phone
25/036/2018 19:36:06 - DEBUG - OUT : CommandID : 6 / Flags : false,true,false / Data length : 34284 / Data CRC32 : 5A 5C DC 51
25/036/2018 19:36:06 - DEBUG - Reading packet from phone
25/036/2018 19:36:06 - DEBUG - IN : CommandID : 6 / Flags : false,false,false / Data length : 0 / Data CRC32 : 00 00 00 00
25/036/2018 19:36:06 - DEBUG - Reading packet from phone
25/036/2018 19:36:07 - DEBUG - IN : CommandID : 1 / Flags : false,false,false / Data length : 845 / Data CRC32 : 4D FF 8A F9
25/036/2018 19:36:07 - DEBUG - Writing packet to phone
25/036/2018 19:36:07 - DEBUG - OUT : CommandID : 1 / Flags : false,true,false / Data length : 0 / Data CRC32 : 00 00 00 00
25/036/2018 19:36:07 - DEBUG - Reading packet from phone
25/036/2018 19:36:07 - DEBUG - IN : CommandID : 1 / Flags : false,false,false / Data length : 845 / Data CRC32 : 4D FF 8A F9
25/036/2018 19:36:07 - DEBUG - PROT_VER="04";VER="R5E006";ID="1240-4121";TYPE="S1_LOADER";NO_SUB_SYSTEMS="0001";UNIQUE_DEV_ID_1="C1FA2EFC539644687FDC6A8C48FF93C71C063491";OTP_DATA_1="030000000000000001000100010001002D48494444454E444154412D1DCC80D7";OTP_LOCK_STATUS_1="LOCKED";IDCODE_1="8500";BOOT_AID="0001";LOADER_AID="0001";HWCONF_AID="0001";ACTIVE_COMP_AID="0001";MEMDEVS="03:00:0045:0000:0090:00ECC000:00080000:00000200";RAM_INFO="FF000018";MAX_PKT_SZ="00010000";AUTH_LEVEL="NONE";AUTH_MET="0001";DEF_SEC="ON";CARD_PRESENT="NO";S1_ROOT="S1_Root_3065,S1_Root_Test_ddab";HW_CONFIG_VER="0005";HW_CONFIG_INFO="R5A";HW_CONFIG="S1_Boot:0001:0013,S1_Loader:0001:000B,S1_SW:0002:0013,S1_Cust:0001:0013,S1_SL:0001:0043,S1_ModemSW:0002:0013,S1_FOTA:0002:002B,S1_ETS:0002:0013,S1_RAMETS:0002:000B";IMEI="35511305333163";PHONE_ID="0000:35511305333163";PROD_ID="0000";GLOB_SEC="0000";
25/036/2018 19:36:07 - DEBUG - After loader command reply (hook) : PROT_VER="04";VER="R5E006";ID="1240-4121";TYPE="S1_LOADER";NO_SUB_SYSTEMS="0001";UNIQUE_DEV_ID_1="C1FA2EFC539644687FDC6A8C48FF93C71C063491";OTP_DATA_1="030000000000000001000100010001002D48494444454E444154412D1DCC80D7";OTP_LOCK_STATUS_1="LOCKED";IDCODE_1="8500";BOOT_AID="0001";LOADER_AID="0001";HWCONF_AID="0001";ACTIVE_COMP_AID="0001";MEMDEVS="03:00:0045:0000:0090:00ECC000:00080000:00000200";RAM_INFO="FF000018";MAX_PKT_SZ="00010000";AUTH_LEVEL="NONE";AUTH_MET="0001";DEF_SEC="ON";CARD_PRESENT="NO";S1_ROOT="S1_Root_3065,S1_Root_Test_ddab";HW_CONFIG_VER="0005";HW_CONFIG_INFO="R5A";HW_CONFIG="S1_Boot:0001:0013,S1_Loader:0001:000B,S1_SW:0002:0013,S1_Cust:0001:0013,S1_SL:0001:0043,S1_ModemSW:0002:0013,S1_FOTA:0002:002B,S1_ETS:0002:0013,S1_RAMETS:0002:000B";IMEI="35511305333163";PHONE_ID="0000:35511305333163";PROD_ID="0000";GLOB_SEC="0000";
25/036/2018 19:36:07 - INFO - Loader : S1_Root_3065 - Version : R5E006 / Boot version : R10E013 / Bootloader status : UNROOTABLE
25/036/2018 19:36:07 - INFO - Max packet size set to 64K
25/036/2018 19:36:07 - INFO - USB buffer size set to 512K
25/036/2018 19:36:08 - INFO - Ending flash session
25/036/2018 19:36:08 - DEBUG - Writing packet to phone
25/036/2018 19:36:08 - DEBUG - OUT : CommandID : 4 / Flags : false,true,false / Data length : 0 / Data CRC32 : 00 00 00 00
25/036/2018 19:36:08 - DEBUG - Reading packet from phone
25/036/2018 19:36:08 - DEBUG - IN : CommandID : 4 / Flags : false,false,false / Data length : 0 / Data CRC32 : 00 00 00 00
25/036/2018 19:36:08 - ERROR - null
25/036/2018 19:36:08 - ERROR - Error flashing. Aborted

The log of xflasher:

-------------------------------------------------------
Xperia Command Line Flasher v23

by Munjeni @ 2014,2017
-------------------------------------------------------
Device path: \\?\usb#vid_0fce&pid_adde#5&345d8d38&0&6#{a5dcbf10-6530-11d2-901f-00c04fb951ed}
Class Description: USB-Controller
Device Instance Id: USB\VID_0FCE&PID_ADDE\5&345D8D38&0&6

- Successfully read 0xd bytes from handle.
Raw input [0xD]:

00000000 00 00 00 01 00 00 00 01 00 00 00 52 59 ...........RY

- Successfully read 0x52 bytes from handle.
Raw input [0x52]:

00000000 50 52 4F 54 5F 56 45 52 3D 22 30 34 22 3B 49 44 PROT_VER="04";ID
00000010 3D 22 31 32 34 30 2D 34 31 32 38 22 3B 56 45 52 ="1240-4128";VER
00000020 3D 22 52 31 30 45 30 31 33 22 3B 54 59 50 45 3D ="R10E013";TYPE=
00000030 22 53 31 5F 42 4F 4F 54 22 3B 41 43 54 49 56 45 "S1_BOOT";ACTIVE
00000040 5F 43 4F 4D 50 5F 41 49 44 3D 22 30 30 30 31 22 _COMP_AID="0001"
00000050 3B 00 ;.

Can't get MAX_PKT_SZ!
-------------------------------------------------------
Xperia Command Line Flasher v23

by Munjeni @ 2014,2017
-------------------------------------------------------
Device path: \\?\usb#vid_0fce&pid_adde#5&345d8d38&0&6#{a5dcbf10-6530-11d2-901f-00c04fb951ed}
Class Description: USB-Controller
Device Instance Id: USB\VID_0FCE&PID_ADDE\5&345D8D38&0&6

Error: file is not a sin!?
Error uploading partition-image.sin!

S1tool says:

PLEASE ATTACH TURNED OFF PHONE NOW

RUNNING S1_BOOT VER "R10E013"
SIMLOCK STATE: "TAMPERED"
LOADER AID: 0001
hash not supported []

Is there something I could try next? Thanks!