[Xperia SP Q&A, Help & Troubleshooting] FeckinL:
I get exactly the same, using Win10 also.

26/052/2015 07:52:26 - INFO - Flashtool Version 0.9.19.7 built on 23-09-2015 22:00:00

26/052/2015 07:52:26 - INFO - Executing search strategies to find proxy selector

26/052/2015 07:52:26 - INFO - No proxy found for IE. Trying next one

26/052/2015 07:52:26 - INFO - Strategy firefox failed trying next one : No Firefox installation found

26/052/2015 07:52:26 - INFO - No proxy found for java. Trying next one

26/052/2015 07:52:27 - INFO - Syncing devices from github

26/052/2015 07:52:27 - INFO - Scanning devices folder for changes.

26/052/2015 07:52:36 - INFO - Pulling changes from github.

26/052/2015 07:52:36 - INFO - Devices sync finished.

26/052/2015 07:52:41 - INFO - Device disconnected

26/053/2015 07:53:11 - INFO - Device connected in flash mode

26/053/2015 07:53:11 - INFO - Selected Bundle for Sony Xperia SP (C5303). FW release : 12.1.A.1.207. Customization : AU

26/053/2015 07:53:11 - INFO - Preparing files for flashing

26/053/2015 07:53:21 - INFO - Please connect your device into flashmode.

26/053/2015 07:53:22 - INFO - Opening device for R/W

26/053/2015 07:53:22 - INFO - Reading device information

26/053/2015 07:53:22 - INFO - Phone ready for flashmode operations.

26/053/2015 07:53:22 - INFO - Opening TA partition 2

26/053/2015 07:53:22 - INFO - Current device : C5303 - YT910CH39W - 1272-3352_R1B - 1272-3352_12.1.A.1.207 - GLOBAL-LTE_12.1.A.1.207

26/053/2015 07:53:22 - INFO - Closing TA partition

26/053/2015 07:53:22 - INFO - Start Flashing

26/053/2015 07:53:22 - INFO - Processing loader.sin

26/053/2015 07:53:22 - INFO - Checking header

26/053/2015 07:53:22 - INFO - Flashing data

26/053/2015 07:53:23 - INFO - Loader : S1_Root_7054 - Version : R5G006 / Boot version : R11B017 / Bootloader status : NOT_ROOTABLE

26/053/2015 07:53:23 - INFO - Max packet size set to 64K

26/053/2015 07:53:23 - INFO - Making a TA backup

26/053/2015 07:53:23 - INFO - Opening TA partition 1

26/053/2015 07:53:23 - INFO - Start Dumping TA partition 1

26/053/2015 07:53:29 - INFO - Finished Dumping TA partition 1

26/053/2015 07:53:29 - INFO - TA partition 1 saved to C:\Users\Superhans\.flashTool\registeredDevices\YT910CH39W\s1ta\2015-09-25_07-53-23\1.ta

26/053/2015 07:53:29 - INFO - Closing TA partition

26/053/2015 07:53:29 - INFO - Opening TA partition 2

26/053/2015 07:53:29 - INFO - Start Dumping TA partition 2

26/053/2015 07:53:31 - INFO - Finished Dumping TA partition 2

26/053/2015 07:53:31 - INFO - TA partition 2 saved to C:\Users\Superhans\.flashTool\registeredDevices\YT910CH39W\s1ta\2015-09-25_07-53-23\2.ta

26/053/2015 07:53:31 - INFO - Closing TA partition

26/053/2015 07:53:32 - INFO - No flash script found. Using 0.9.18 flash engine

26/053/2015 07:53:32 - INFO - Opening TA partition 2

26/053/2015 07:53:33 - INFO - Writing TA unit 00002774. Value : [01]

26/053/2015 07:53:33 - INFO - Closing TA partition

26/053/2015 07:53:34 - INFO - Processing partition-image.sin

26/053/2015 07:53:34 - INFO - Checking header

26/053/2015 07:53:34 - INFO - Flashing data

26/053/2015 07:53:38 - INFO - Opening TA partition 2

26/053/2015 07:53:38 - INFO - Closing TA partition

26/053/2015 07:53:39 - INFO - Opening TA partition 2

26/053/2015 07:53:39 - INFO - Processing amss_fsg.sin

26/053/2015 07:53:39 - INFO - Checking header

26/053/2015 07:53:39 - INFO - Flashing data

26/053/2015 07:53:41 - INFO - Processing amss_fs_1.sin

26/053/2015 07:53:41 - INFO - Checking header

26/053/2015 07:53:41 - INFO - Flashing data

26/053/2015 07:53:43 - INFO - Processing amss_fs_2.sin

26/053/2015 07:53:43 - INFO - Checking header

26/053/2015 07:53:43 - INFO - Flashing data

26/053/2015 07:53:44 - INFO - Processing apps_log.sin

26/053/2015 07:53:44 - INFO - Checking header

26/053/2015 07:53:44 - INFO - Flashing data

26/053/2015 07:53:46 - INFO - Processing cache.sin

26/053/2015 07:53:47 - INFO - Checking header

26/053/2015 07:53:47 - INFO - Flashing data

26/053/2015 07:53:51 - INFO - Processing fotakernel.sin

26/053/2015 07:53:51 - INFO - Checking header

26/053/2015 07:53:51 - INFO - Flashing data

26/053/2015 07:53:54 - INFO - Processing kernel.sin

26/053/2015 07:53:54 - INFO - Checking header

26/053/2015 07:53:54 - INFO - Flashing data

26/053/2015 07:53:59 - INFO - Closing TA partition

26/054/2015 07:54:00 - INFO - Opening TA partition 2

26/054/2015 07:54:00 - INFO - Processing system.sin

26/054/2015 07:54:00 - INFO - Checking header

26/054/2015 07:54:01 - ERROR - Processing of system.sin finished with errors.

26/054/2015 07:54:01 - INFO - Ending flash session

26/054/2015 07:54:01 - ERROR -

26/054/2015 07:54:01 - ERROR - Error flashing. Aborted

26/054/2015 07:54:01 - INFO - Flashing finished.

26/054/2015 07:54:01 - INFO - Please unplug and start your phone

26/054/2015 07:54:01 - INFO - For flashtool, Unknown Sources and Debugging must be checked in phone settings

26/054/2015 07:54:01 - INFO - Device connected in flash mode

26/054/2015 07:54:02 - INFO - Device disconnected

26/054/2015 07:54:05 - INFO - Device connected in flash mode

26/054/2015 07:54:48 - INFO - Device disconnected

26/055/2015 07:55:53 - INFO - Device connected in flash mode

26/056/2015 07:56:35 - INFO - Device disconnected

26/057/2015 07:57:40 - INFO - Device connected in flash mode

26/058/2015 07:58:23 - INFO - Device disconnected

26/059/2015 07:59:27 - INFO - Device connected in flash mode

26/059/2015 07:59:50 - INFO - Device Status: NOT FOUND

26/059/2015 07:59:54 - ERROR - This action needs a connected device in ADB mode

26/059/2015 07:59:59 - INFO - Now plug your device in Fastboot Mode

26/000/2015 08:00:05 - INFO - Device disconnected

26/000/2015 08:00:28 - INFO - Device connected in flash mode

26/000/2015 08:00:50 - INFO - Device disconnected

26/000/2015 08:00:52 - INFO - Device connected in flash mode

26/000/2015 08:00:57 - INFO - Failed

26/001/2015 08:01:01 - INFO - Now plug your device in Fastboot Mode

26/001/2015 08:01:04 - INFO - Failed

26/001/2015 08:01:06 - INFO - Now plug your device in Fastboot Mode

26/001/2015 08:01:10 - INFO - Device disconnected

26/001/2015 08:01:32 - INFO - Device connected in flash mode

26/001/2015 08:01:45 - INFO - Device disconnected

26/002/2015 08:02:13 - INFO - Failed

26/002/2015 08:02:17 - INFO - Now plug your device in Fastboot Mode

26/002/2015 08:02:21 - INFO - Failed

26/002/2015 08:02:40 - INFO - Now plug your device in Fastboot Mode

26/003/2015 08:03:02 - INFO - Failed

26/003/2015 08:03:46 - INFO - Device connected in flash mode

26/003/2015 08:03:50 - INFO - Selected Bundle for Sony Xperia SP (C5303). FW release : 12.1.A.1.207. Customization : AU

26/003/2015 08:03:50 - INFO - Preparing files for flashing

26/003/2015 08:03:58 - INFO - Please connect your device into flashmode.

26/003/2015 08:03:59 - INFO - Opening device for R/W

26/003/2015 08:03:59 - INFO - Reading device information

26/003/2015 08:03:59 - INFO - Phone ready for flashmode operations.

26/003/2015 08:03:59 - INFO - Opening TA partition 2

26/003/2015 08:03:59 - INFO - Current device : C5303 - YT910CH39W - 1272-3352_R1B - 1272-3352_12.1.A.1.207 - GLOBAL-LTE_12.1.A.1.207

26/003/2015 08:03:59 - INFO - Closing TA partition

26/003/2015 08:03:59 - INFO - Start Flashing

26/003/2015 08:03:59 - INFO - Processing loader.sin

26/003/2015 08:03:59 - INFO - Checking header

26/003/2015 08:03:59 - INFO - Flashing data

26/004/2015 08:04:00 - INFO - Loader : S1_Root_7054 - Version : R5G006 / Boot version : R11B017 / Bootloader status : NOT_ROOTABLE

26/004/2015 08:04:00 - INFO - Max packet size set to 64K

26/004/2015 08:04:00 - INFO - Making a TA backup

26/004/2015 08:04:00 - INFO - Opening TA partition 1

26/004/2015 08:04:00 - INFO - Start Dumping TA partition 1

26/004/2015 08:04:06 - INFO - Finished Dumping TA partition 1

26/004/2015 08:04:06 - INFO - TA partition 1 saved to C:\Users\Superhans\.flashTool\registeredDevices\YT910CH39W\s1ta\2015-09-26_08-04-00\1.ta

26/004/2015 08:04:06 - INFO - Closing TA partition

26/004/2015 08:04:06 - INFO - Opening TA partition 2

26/004/2015 08:04:07 - INFO - Start Dumping TA partition 2

26/004/2015 08:04:08 - INFO - Finished Dumping TA partition 2

26/004/2015 08:04:09 - INFO - TA partition 2 saved to C:\Users\Superhans\.flashTool\registeredDevices\YT910CH39W\s1ta\2015-09-26_08-04-00\2.ta

26/004/2015 08:04:09 - INFO - Closing TA partition

26/004/2015 08:04:10 - INFO - No flash script found. Using 0.9.18 flash engine

26/004/2015 08:04:10 - INFO - Opening TA partition 2

26/004/2015 08:04:10 - INFO - Writing TA unit 00002774. Value : [01]

26/004/2015 08:04:10 - INFO - Closing TA partition

26/004/2015 08:04:11 - INFO - Processing partition-image.sin

26/004/2015 08:04:11 - INFO - Checking header

26/004/2015 08:04:11 - INFO - Flashing data

26/004/2015 08:04:15 - INFO - Opening TA partition 2

26/004/2015 08:04:15 - INFO - Closing TA partition

26/004/2015 08:04:16 - INFO - Opening TA partition 2

26/004/2015 08:04:16 - INFO - Processing amss_fsg.sin

26/004/2015 08:04:16 - INFO - Checking header

26/004/2015 08:04:16 - INFO - Flashing data

26/004/2015 08:04:19 - INFO - Processing amss_fs_1.sin

26/004/2015 08:04:19 - INFO - Checking header

26/004/2015 08:04:19 - INFO - Flashing data

26/004/2015 08:04:21 - INFO - Processing amss_fs_2.sin

26/004/2015 08:04:21 - INFO - Checking header

26/004/2015 08:04:21 - INFO - Flashing data

26/004/2015 08:04:22 - INFO - Processing apps_log.sin

26/004/2015 08:04:22 - INFO - Checking header

26/004/2015 08:04:22 - INFO - Flashing data

26/004/2015 08:04:24 - INFO - Processing cache.sin

26/004/2015 08:04:24 - INFO - Checking header

26/004/2015 08:04:24 - INFO - Flashing data

26/004/2015 08:04:28 - INFO - Processing fotakernel.sin

26/004/2015 08:04:28 - INFO - Checking header

26/004/2015 08:04:29 - INFO - Flashing data

26/004/2015 08:04:32 - INFO - Processing kernel.sin

26/004/2015 08:04:32 - INFO - Checking header

26/004/2015 08:04:32 - INFO - Flashing data

26/004/2015 08:04:37 - INFO - Closing TA partition

26/004/2015 08:04:37 - INFO - Opening TA partition 2

26/004/2015 08:04:37 - INFO - Processing system.sin

26/004/2015 08:04:37 - INFO - Checking header

26/004/2015 08:04:38 - ERROR - Processing of system.sin finished with errors.

26/004/2015 08:04:38 - INFO - Ending flash session

26/004/2015 08:04:38 - ERROR -

26/004/2015 08:04:38 - ERROR - Error flashing. Aborted

26/004/2015 08:04:38 - INFO - Flashing finished.

26/004/2015 08:04:38 - INFO - Please unplug and start your phone

26/004/2015 08:04:38 - INFO - For flashtool, Unknown Sources and Debugging must be checked in phone settings

26/004/2015 08:04:39 - INFO - Device connected in flash mode

26/004/2015 08:04:40 - INFO - Device disconnected

26/004/2015 08:04:42 - INFO - Device connected in flash mode

26/005/2015 08:05:25 - INFO - Device disconnected

26/006/2015 08:06:30 - INFO - Device connected in flash mode

26/007/2015 08:07:13 - INFO - Device disconnected