[Sony Xperia S, Acro S, Ion] itstherealone:
I tried to flash the stock ftf another time using flashtool after i managed to get in flash mode by turn off the mobile thru TWRP after unplugging the batterie for some minutes.

the log says flashing worked:

09/029/2017 00:29:48 - INFO - Flashtool Version 0.9.18.3 built on 10-20-2014 08:00:00

09/029/2017 00:29:48 - INFO - Executing search strategies to find proxy selector

09/029/2017 00:29:49 - INFO - No proxy found for IE. Trying next one

09/029/2017 00:29:49 - INFO - Strategy firefox failed trying next one : No Firefox installation found

09/029/2017 00:29:49 - INFO - No proxy found for java. Trying next one

09/029/2017 00:29:49 - INFO - Syncing devices from github

09/029/2017 00:29:50 - INFO - Scanning devices folder for changes.

09/030/2017 00:30:21 - INFO - Pulling changes from github.

09/030/2017 00:30:21 - INFO - Devices sync finished.

09/030/2017 00:30:32 - INFO - Device disconnected

09/035/2017 00:35:12 - INFO - Device connected with USB debugging off

09/035/2017 00:35:14 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/015/2017 01:15:02 - INFO - Device disconnected

09/015/2017 01:15:08 - INFO - Device connected with USB debugging off

09/015/2017 01:15:08 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/015/2017 01:15:16 - INFO - Device disconnected

09/015/2017 01:15:17 - INFO - Device connected with USB debugging off

09/015/2017 01:15:17 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/015/2017 01:15:25 - INFO - Device disconnected

09/015/2017 01:15:27 - INFO - Device connected with USB debugging off

09/015/2017 01:15:27 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/015/2017 01:15:32 - INFO - Device disconnected

09/015/2017 01:15:34 - INFO - Device connected with USB debugging off

09/015/2017 01:15:34 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/015/2017 01:15:37 - INFO - Device disconnected

09/016/2017 01:16:09 - ERROR - Drivers need to be installed for connected device.

09/016/2017 01:16:09 - ERROR - You can find them in the drivers folder of Flashtool.

09/016/2017 01:16:14 - INFO - Device connected with USB debugging off

09/016/2017 01:16:14 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/035/2017 01:35:51 - INFO - Device disconnected

09/036/2017 01:36:20 - INFO - Device connected in flash mode

09/036/2017 01:36:38 - ERROR - root : this bundle is not valid

09/036/2017 01:36:39 - INFO - Selected Bundle for Sony Xperia S (LT26i). FW release : 6.2.B.1.96. Customization : Telekom.de_DE

09/036/2017 01:36:39 - INFO - Preparing files for flashing

09/037/2017 01:37:03 - INFO - Device disconnected

09/037/2017 01:37:25 - ERROR - Drivers need to be installed for connected device.

09/037/2017 01:37:25 - ERROR - You can find them in the drivers folder of Flashtool.

09/037/2017 01:37:28 - INFO - Device connected with USB debugging off

09/037/2017 01:37:28 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/037/2017 01:37:42 - INFO - Device disconnected

09/037/2017 01:37:45 - INFO - Please connect your device into flashmode.

09/038/2017 01:38:51 - INFO - Device connected with USB debugging off

09/038/2017 01:38:51 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/039/2017 01:39:03 - INFO - Device disconnected

09/039/2017 01:39:04 - INFO - Device connected with USB debugging off

09/039/2017 01:39:04 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/040/2017 01:40:24 - INFO - Device disconnected

09/041/2017 01:41:36 - INFO - Device connected with USB debugging off

09/041/2017 01:41:36 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/042/2017 01:42:39 - ERROR - Drivers need to be installed for connected device.

09/042/2017 01:42:39 - ERROR - You can find them in the drivers folder of Flashtool.

09/048/2017 01:48:49 - INFO - Device disconnected

09/053/2017 01:53:24 - INFO - Device connected with USB debugging off

09/053/2017 01:53:44 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/057/2017 01:57:33 - INFO - Device disconnected

09/047/2017 02:47:37 - INFO - Device connected in flash mode

09/047/2017 02:47:59 - INFO - Device disconnected

09/048/2017 02:48:07 - INFO - Opening device for R/W

09/048/2017 02:48:28 - INFO - Reading device information

09/048/2017 02:48:30 - INFO - Unable to read from phone after having opened it.

09/048/2017 02:48:30 - INFO - trying to continue anyway

09/048/2017 02:48:30 - INFO - Start Flashing

09/048/2017 02:48:31 - INFO - Processing loader.sin

09/048/2017 02:48:31 - INFO - Checking header

09/048/2017 02:48:31 - ERROR - Processing of loader.sin finished with errors.

09/048/2017 02:48:31 - INFO - Ending flash session

09/048/2017 02:48:31 - ERROR - Error in processHeader : 1 : Unzulässige Funktion.

09/048/2017 02:48:31 - ERROR - Error flashing. Aborted

09/048/2017 02:48:32 - INFO - Device connected with USB debugging off

09/048/2017 02:48:32 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/049/2017 02:49:17 - INFO - Device disconnected

09/056/2017 02:56:32 - INFO - Device connected with USB debugging off

09/056/2017 02:56:32 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/012/2017 03:12:51 - INFO - Device disconnected

09/043/2017 13:43:17 - INFO - Selected null

09/043/2017 13:43:18 - INFO - Flash canceled

09/044/2017 13:44:32 - ERROR - root : this bundle is not valid

09/044/2017 13:44:32 - INFO - Selected Bundle for Sony Xperia S (LT26i). FW release : 6.2.B.1.96. Customization : World

09/044/2017 13:44:32 - INFO - Preparing files for flashing

09/044/2017 13:44:43 - INFO - Please connect your device into flashmode.

09/040/2017 14:40:11 - INFO - Flash canceled

09/012/2017 16:12:57 - ERROR - root : this bundle is not valid

09/012/2017 16:12:57 - INFO - Selected Bundle for Sony Xperia S (LT26i). FW release : 6.2.B.1.96. Customization : World

09/012/2017 16:12:57 - INFO - Preparing files for flashing

09/013/2017 16:13:10 - INFO - Please connect your device into flashmode.

09/035/2017 16:35:21 - INFO - Device connected with USB debugging off

09/035/2017 16:35:21 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/035/2017 16:35:46 - INFO - Device disconnected

09/035/2017 16:35:46 - INFO - Device connected with USB debugging off

09/035/2017 16:35:46 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/050/2017 16:50:33 - INFO - Device disconnected

09/008/2017 19:08:42 - INFO - Device connected with USB debugging off

09/008/2017 19:08:58 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/033/2017 19:33:26 - INFO - Device disconnected

09/017/2017 21:17:22 - INFO - Device connected with USB debugging off

09/018/2017 21:18:03 - INFO - For 2011 devices line, be sure you are not in MTP mode

09/033/2017 21:33:38 - INFO - Device disconnected

10/039/2017 00:39:59 - INFO - Device connected with USB debugging off

10/040/2017 00:40:13 - INFO - For 2011 devices line, be sure you are not in MTP mode

10/041/2017 00:41:58 - INFO - Device disconnected

10/051/2017 00:51:02 - INFO - Device connected with USB debugging off

10/051/2017 00:51:02 - INFO - For 2011 devices line, be sure you are not in MTP mode

10/011/2017 01:11:01 - INFO - Device disconnected

10/013/2017 15:13:01 - INFO - Device connected in flash mode

10/013/2017 15:13:10 - INFO - Opening device for R/W

10/013/2017 15:13:16 - INFO - Reading device information

10/013/2017 15:13:24 - INFO - Phone ready for flashmode operations.

10/013/2017 15:13:24 - INFO - Current device : Unknown: Jul 25 2013/14:37:45 - BX9037CLP9 - Unknown: Jul 25 2013/14:37:45 - Unknown: Jul 25 2013/14:37:45 - Unknown: Jul 25 2013/14:37:45

10/013/2017 15:13:27 - INFO - Start Flashing

10/013/2017 15:13:27 - INFO - Processing loader.sin

10/013/2017 15:13:27 - INFO - Checking header

10/013/2017 15:13:27 - INFO - Flashing data

10/013/2017 15:13:28 - INFO - Processing of loader.sin finished.

10/013/2017 15:13:29 - INFO - Loader : S1_Root_de72 - Version : R5C011 / Boot version : R10C008 / Bootloader status : ROOTED

10/013/2017 15:13:31 - INFO - Disabling final data verification check

10/013/2017 15:13:31 - INFO - Processing partition-image.sin

10/013/2017 15:13:31 - INFO - Checking header

10/013/2017 15:13:31 - INFO - Flashing data

10/013/2017 15:13:31 - INFO - Processing of partition-image.sin finished.

10/013/2017 15:13:31 - INFO - No bootdelivery to send

10/013/2017 15:13:32 - INFO - Processing kernel.sin

10/013/2017 15:13:32 - INFO - Checking header

10/013/2017 15:13:32 - INFO - Flashing data

10/013/2017 15:13:35 - INFO - Processing of kernel.sin finished.

10/013/2017 15:13:37 - INFO - Processing fotakernel.sin

10/013/2017 15:13:37 - INFO - Checking header

10/013/2017 15:13:37 - INFO - Flashing data

10/013/2017 15:13:39 - INFO - Processing of fotakernel.sin finished.

10/013/2017 15:13:40 - INFO - Processing amss_fs_2.sin

10/013/2017 15:13:40 - INFO - Checking header

10/013/2017 15:13:40 - INFO - Flashing data

10/013/2017 15:13:40 - INFO - Processing of amss_fs_2.sin finished.

10/013/2017 15:13:40 - INFO - Processing amss_fs_1.sin

10/013/2017 15:13:40 - INFO - Checking header

10/013/2017 15:13:40 - INFO - Flashing data

10/013/2017 15:13:40 - INFO - Processing of amss_fs_1.sin finished.

10/013/2017 15:13:40 - INFO - Processing amss_fsg.sin

10/013/2017 15:13:40 - INFO - Checking header

10/013/2017 15:13:40 - INFO - Flashing data

10/013/2017 15:13:40 - INFO - Processing of amss_fsg.sin finished.

10/013/2017 15:13:40 - INFO - Processing apps_log.sin

10/013/2017 15:13:40 - INFO - Checking header

10/013/2017 15:13:41 - INFO - Flashing data

10/013/2017 15:13:41 - INFO - Processing of apps_log.sin finished.

10/013/2017 15:13:41 - INFO - Processing cache.sin

10/013/2017 15:13:41 - INFO - Checking header

10/013/2017 15:13:41 - INFO - Flashing data

10/013/2017 15:13:43 - INFO - Processing of cache.sin finished.

10/013/2017 15:13:43 - INFO - Processing system.sin

10/013/2017 15:13:43 - INFO - Checking header

10/013/2017 15:13:43 - INFO - Flashing data

10/016/2017 15:16:03 - INFO - Processing of system.sin finished.

10/016/2017 15:16:05 - INFO - Processing userdata.sin

10/016/2017 15:16:05 - INFO - Checking header

10/016/2017 15:16:05 - INFO - Flashing data

10/016/2017 15:16:11 - INFO - Processing of userdata.sin finished.

10/016/2017 15:16:11 - WARN - This file is ignored : simlock.ta

10/016/2017 15:16:12 - INFO - Ending flash session

10/016/2017 15:16:12 - INFO - Flashing finished.

10/016/2017 15:16:12 - INFO - Please unplug and start your phone

10/016/2017 15:16:12 - INFO - For flashtool, Unknown Sources and Debugging must be checked in phone settings

10/016/2017 15:16:12 - INFO - Device connected in flash mode

10/017/2017 15:17:01 - INFO - Device disconnected

now i'm waiting for the device to start up plugged in to a wall charger (no led light). at the moment it stucks with just the backlight on after sony logo...