[Sony Xperia M] cabriofahrer:
I have an Xperia M C1905 and it is unusable due to endless popups. Due to these popups I cannot even perform a factory reset in the settings menu, the phone will eventually hang. Same thing if I try to boot into safe mode by pressing the power and volume down button simultaneously until the boot process is complete. Due to endless popups it is impossible to get anywhere and e.g. uninstall apps that might be part of the problem. I know that I would need 15.4.A.1.9-R6C / CDA 1274-8220 / Vodafone ES (Spain). I am unable to find that specific .ftf, all I have found here is a customised UK-Version and a so called CE-Version here. Anyway, I thought I might give those FTF's a try with flashtool (Under Linux Fedora 25), but that does not work. Here is the output from the UK-Version (CE was practically the same):

05/011/2017 12:11:03 - INFO - <- This level is successfully initialized
05/011/2017 12:11:09 - INFO - Flashtool Version 0.9.23.1 built on 11-01-2017 15:12:00
05/011/2017 12:11:09 - INFO - libusb version 1.0.20
05/011/2017 12:11:09 - INFO - Executing search strategies to find proxy selector
05/011/2017 12:11:09 - INFO - No proxy found for gnome. Trying next one
05/011/2017 12:11:09 - INFO - No proxy found for kde. Trying next one
05/011/2017 12:11:09 - INFO - Strategy firefox failed trying next one : No Firefox installation found
05/011/2017 12:11:09 - INFO - Syncing devices from github
05/011/2017 12:11:10 - INFO - Opening devices repository.
05/011/2017 12:11:12 - INFO - Scanning devices folder for changes.
05/011/2017 12:11:28 - INFO - Pulling changes from github.
05/011/2017 12:11:29 - INFO - Quietly closing devices repository.
05/011/2017 12:11:29 - INFO - Devices sync finished.
05/011/2017 12:11:29 - INFO - Loading devices database
05/011/2017 12:11:30 - INFO - Loaded 98 devices
05/011/2017 12:11:30 - INFO - Starting phone detection
05/011/2017 12:11:33 - INFO - Device disconnected
05/013/2017 12:13:05 - INFO - Selected Bundle for Sony Xperia M(C1905). FW release : 15.4.A.1.9. Customization : Customized UK
05/013/2017 12:13:05 - INFO - Preparing files for flashing
05/014/2017 12:14:38 - INFO - Please connect your device into flashmode.
05/021/2017 12:21:21 - INFO - Device connected in flash mode
05/021/2017 12:21:21 - INFO - Opening device for R/W
05/021/2017 12:21:21 - INFO - Device ready for R/W.
05/021/2017 12:21:21 - INFO - Reading device information
05/021/2017 12:21:21 - INFO - Phone ready for flashmode operations.
05/021/2017 12:21:21 - INFO - Opening TA partition 2
05/021/2017 12:21:21 - INFO - Current device : C1905 - YT910LSGX1 - 1274-8220_R6C - 1272-2325_15.4.A.1.9 - WORLD-i_15.4.A.1.9
05/021/2017 12:21:21 - INFO - Closing TA partition
05/021/2017 12:21:21 - INFO - Start Flashing
05/021/2017 12:21:21 - INFO - No loader in the bundle. Searching for one
05/021/2017 12:21:21 - INFO - Processing loader.sin
05/021/2017 12:21:21 - INFO - Checking header
05/021/2017 12:21:21 - INFO - Flashing data
05/021/2017 12:21:22 - INFO - Loader : S1_Root_dbe9 - Version : loader_MSM8X30_10 / Boot version : S1_Boot_MSM_8227_5 / Bootloader status : NOT_ROOTABLE
05/021/2017 12:21:22 - INFO - Max packet size set to 512K
05/021/2017 12:21:22 - INFO - USB buffer size set to 512K
05/021/2017 12:21:53 - INFO - Parsing boot delivery
05/021/2017 12:21:58 - INFO - No flash script found. Using 0.9.18 flash engine
05/021/2017 12:21:58 - INFO - Opening TA partition 2
05/021/2017 12:21:59 - INFO - Writing TA unit 00002774. Value : 01
05/021/2017 12:21:59 - INFO - Closing TA partition
05/021/2017 12:21:59 - INFO - Processing partition.sin
05/021/2017 12:21:59 - INFO - Checking header
05/021/2017 12:21:59 - INFO - Flashing data
05/021/2017 12:21:59 - INFO - Phone boot version : S1_Boot_MSM_8227_5. Boot delivery version : S1_Boot_MSM_8227_5
05/021/2017 12:21:59 - INFO - Boot delivery up to date. Nothing to do
05/021/2017 12:21:59 - INFO - Opening TA partition 2
05/022/2017 12:22:04 - INFO - Closing TA partition
05/022/2017 12:22:06 - INFO - Opening TA partition 2
05/022/2017 12:22:06 - INFO - Processing cache.sin
05/022/2017 12:22:06 - INFO - Checking header
05/022/2017 12:22:06 - INFO - Flashing data
05/022/2017 12:22:06 - ERROR - Processing of cache.sin finished with errors.
05/022/2017 12:22:06 - INFO - Ending flash session
05/022/2017 12:22:06 - ERROR -
05/022/2017 12:22:06 - ERROR - Error flashing. Aborted
05/022/2017 12:22:06 - INFO - Flashing finished.
05/022/2017 12:22:06 - INFO - Please unplug and start your phone
05/022/2017 12:22:06 - INFO - For flashtool, Unknown Sources and Debugging must be checked in phone settings
05/022/2017 12:22:07 - INFO - Device connected in flash mode
05/022/2017 12:22:08 - INFO - Device disconnected
05/022/2017 12:22:19 - INFO - Device connected with USB debugging off
05/022/2017 12:22:19 - INFO - For 2011 devices line, be sure you are not in MTP mode
05/026/2017 12:26:41 - INFO - Device disconnected

Is this error due to the FTF not being the exact one actually needed (Vodafone ES)? I do have USB debugging turned on in my settings. I also tried flashing again excluding "cache.sin" in the flashtool options, but then next time the error would be given with "kernel.sin".