[Xperia Z1 Compact Android Development] swissly:
I tried a different PC, different cable but still no success. "S1Boot Download" device shows up correctly on both PC with driver installed and no yellow triangle.

I have downloaded the stock firmware with XperiFirm. But flashtool does not work, this is the output:

24/011/2018 19:11:12 - INFO - (FlashJob.java:36) - Please connect your device into flashmode.
24/012/2018 19:12:50 - INFO - (MainSWT.java:158) - Device connected in flash mode
24/012/2018 19:12:51 - INFO - (USBFlashWin32.java:32) - Opening device for R/W
24/012/2018 19:12:51 - INFO - (MainSWT.java:158) - Device connected in flash mode
24/012/2018 19:12:51 - INFO - (X10flash.java:707) - Start Flashing
24/012/2018 19:12:51 - INFO - (X10flash.java:306) - Processing loader.sin
24/012/2018 19:12:51 - INFO - (X10flash.java:288) - Checking header
24/012/2018 19:12:51 - ERROR - (X10flash.java:326) - Processing of loader.sin finished with errors.
24/012/2018 19:12:51 - INFO - (X10flash.java:910) - Ending flash session
24/012/2018 19:12:51 - ERROR - (X10flash.java:735) - null
24/012/2018 19:12:51 - ERROR - (X10flash.java:736) - Error flashing. Aborted
24/012/2018 19:12:52 - INFO - (MainSWT.java:158) - Device connected in flash mode

And xflasher does not work either, this is the log file output:

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

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

ERROR: GetOverlapped_in_Result: failed with error code 87 as follows:
Wrong Parameter.

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

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

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

This is the code in xflasher.bat

@setlocal enableextensions enabledelayedexpansion
:::upload loader.sin (please change this to loader.sin in case your device uses loader.sin)
::: (please change to noloader in case your device don't use loader! )
xflasher.exe loader.sin 0 0 0FCE ADDE 0 0 >xflasher.log & if %ERRORLEVEL% neq 0 ( exit %ERRORLEVEL% )

:::flash sin files
xflasher.exe partition-image.sin 0 0 0FCE ADDE 0 0 >>xflasher.log & if %ERRORLEVEL% neq 0 ( exit %ERRORLEVEL% )
xflasher.exe fotakernel.sin 0 0 0FCE ADDE 0 0 >>xflasher.log & if %ERRORLEVEL% neq 0 ( exit %ERRORLEVEL% )
xflasher.exe kernel.sin 0 0 0FCE ADDE 0 0 >>xflasher.log & if %ERRORLEVEL% neq 0 ( exit %ERRORLEVEL% )
xflasher.exe modem.sin 0 0 0FCE ADDE 0 0 >>xflasher.log & if %ERRORLEVEL% neq 0 ( exit %ERRORLEVEL% )
xflasher.exe modemfs.sin 0 0 0FCE ADDE 0 0 >>xflasher.log & if %ERRORLEVEL% neq 0 ( exit %ERRORLEVEL% )
xflasher.exe prcmu.sin 0 0 0FCE ADDE 0 0 >>xflasher.log & if %ERRORLEVEL% neq 0 ( exit %ERRORLEVEL% )
xflasher.exe system.sin 0 0 0FCE ADDE 0 0 >>xflasher.log & if %ERRORLEVEL% neq 0 ( exit %ERRORLEVEL% )
xflasher.exe cache.sin 0 0 0FCE ADDE 0 0 >>xflasher.log & if %ERRORLEVEL% neq 0 ( exit %ERRORLEVEL% )

Any other ideas what I could try?