Flashtool: 'Error flashing. Aborted' with no other error message.

Hi everyone.
I'm really confused about this. I'm trying to downgrade the firmware on my Z3c to then go on and root but I'm stuck at the very beginning.

I installed Flashtool and the drivers (had to deactivate driver signature verification for them to install correctly, but now all of them are installed). I installed the Sony PC companion with the support zone addon (not sure it makes a difference).
I downloaded the firmware from 2 different sources.
Developer options and usb debugging are on.
I charged the phone.
I tried with the latest Flashtool version anb the previous one.

But all the times I get the following error message. It basically starts printing out stuff and after a few seconds it says error, aborted. There is no other error message and this really drives me crazy because I have no idea what's wrong. Searching online results in people asking for help when there is at least another error message.
In this case there is nothing.

In the log below, note the time for every message. In the phone nothing changed, it still boots up normally with the 5.1.1 Sony firmware.
I even tried to change the usb cable but it's always the same.

It would be great if some of you were able to pinpoint the source of this problem. I've had and loved this phone for the last few months and I finally decided to root it, but being stuck at the very beginning I'm kind of disappointed at the process.

Thanks


--- log below ---


30/025/2015 10:25:48 - INFO - Selected Bundle for D5803. FW release : 23.0.A.2.9. Customization : Generic GLOBAL_1286-9917
30/025/2015 10:25:48 - INFO - Preparing files for flashing
30/025/2015 10:25:59 - INFO - Please connect your device into flashmode.
30/026/2015 10:26:05 - INFO - Device connected in flash mode
30/026/2015 10:26:05 - INFO - Opening device for R/W
30/026/2015 10:26:05 - INFO - Reading device information
30/026/2015 10:26:06 - INFO - Phone ready for flashmode operations.
30/026/2015 10:26:06 - INFO - Opening TA partition 2
30/026/2015 10:26:06 - INFO - Current device : D5803 - YT910VXETU - 1289-4008_R3C - 1285-6521_23.4.A.1.200 - GLOBAL-LTE_23.4.A.1.200
30/026/2015 10:26:06 - INFO - Closing TA partition
30/026/2015 10:26:06 - INFO - Start Flashing
30/026/2015 10:26:06 - INFO - Processing loader.sin
30/026/2015 10:26:06 - INFO - Checking header
30/026/2015 10:26:06 - INFO - Flashing data
30/026/2015 10:26:07 - INFO - Loader : S1_Root_9c84 - Version : MSM8974AB_22 / Boot version : S1_Boot_MSM8974AC_LA3.0_L_37 / Bootloader status : ROOTABLE
30/026/2015 10:26:07 - INFO - Max packet size set to 512K
30/026/2015 10:26:08 - INFO - Making a TA backup
30/026/2015 10:26:08 - INFO - Opening TA partition 1
30/026/2015 10:26:08 - INFO - Start Dumping TA partition 1
30/026/2015 10:26:08 - INFO - Finished Dumping TA partition 1
30/026/2015 10:26:08 - INFO - TA partition 1 saved to C:\Users\peruz\.flashTool\registeredDevices\YT910V XETU\s1ta15-09-30_10-26-08.ta
30/026/2015 10:26:08 - INFO - Closing TA partition
30/026/2015 10:26:08 - INFO - Opening TA partition 2
30/026/2015 10:26:08 - INFO - Start Dumping TA partition 2
30/026/2015 10:26:09 - INFO - Finished Dumping TA partition 2
30/026/2015 10:26:09 - INFO - TA partition 2 saved to C:\Users\peruz\.flashTool\registeredDevices\YT910V XETU\s1ta15-09-30_10-26-08.ta
30/026/2015 10:26:09 - INFO - Closing TA partition
30/026/2015 10:26:09 - INFO - Parsing boot delivery
30/026/2015 10:26:09 - INFO - Ending flash session
30/026/2015 10:26:09 - ERROR -
30/026/2015 10:26:09 - ERROR - Error flashing. Aborted
30/026/2015 10:26:09 - INFO - Device connected in flash mode
30/026/2015 10:26:10 - INFO - Device disconnected