[Xperia Z Q&A, Help & Troubleshooting] coenv:
The strange thing is, I get no error messages at all.

This is the log I get when I flash the generic firmware with flashtool:

20/027/2013 16:27:22 - INFO - Selected C6603 / 10.3.A.0.423 / Generic NL

20/027/2013 16:27:22 - INFO - Preparing files for flashing

20/027/2013 16:27:35 - INFO - Device disconnected

20/027/2013 16:27:46 - INFO - Device connected in flash mode

20/029/2013 16:29:01 - INFO - Please connect your device into flashmode.

20/029/2013 16:29:02 - INFO - Opening device for R/W

20/029/2013 16:29:02 - INFO - Device connected in flash mode

20/029/2013 16:29:03 - INFO - Reading device information

20/029/2013 16:29:03 - INFO - Phone ready for flashmode operations.

20/029/2013 16:29:03 - INFO - Current device : C6603 - CB5A1PNK4R - 1270-7280_R2B - 1269-5309_10.3.1.A.2.67 - GLOBAL-LTE_10.3.1.A.2.67

20/029/2013 16:29:03 - INFO - Start Flashing

20/029/2013 16:29:03 - INFO - Processing loader.sin

20/029/2013 16:29:03 - INFO - Checking header

20/029/2013 16:29:03 - INFO - Flashing data

20/029/2013 16:29:03 - INFO - Loader : S1_Root_5ca3 - Version : APQ8064_39 / Boot version : S1_Boot_Lagan_1.1.1_1 / Bootloader status : ROOTED

20/029/2013 16:29:04 - INFO - Processing partition-image.sin

20/029/2013 16:29:04 - INFO - Checking header

20/029/2013 16:29:04 - INFO - Flashing data

20/029/2013 16:29:04 - INFO - Processing kernel.sin

20/029/2013 16:29:04 - INFO - Checking header

20/029/2013 16:29:04 - INFO - Flashing data

20/029/2013 16:29:08 - INFO - Processing rpm.sin

20/029/2013 16:29:08 - INFO - Checking header

20/029/2013 16:29:08 - INFO - Flashing data

20/029/2013 16:29:08 - INFO - Processing fotakernel.sin

20/029/2013 16:29:08 - INFO - Checking header

20/029/2013 16:29:08 - INFO - Flashing data

20/029/2013 16:29:09 - INFO - Processing amss_fs_3.sin

20/029/2013 16:29:09 - INFO - Checking header

20/029/2013 16:29:09 - INFO - Flashing data

20/029/2013 16:29:10 - INFO - Processing amss_fs_2.sin

20/029/2013 16:29:10 - INFO - Checking header

20/029/2013 16:29:10 - INFO - Flashing data

20/029/2013 16:29:10 - INFO - Processing amss_fs_1.sin

20/029/2013 16:29:10 - INFO - Checking header

20/029/2013 16:29:10 - INFO - Flashing data

20/029/2013 16:29:10 - INFO - Processing cache.sin

20/029/2013 16:29:10 - INFO - Checking header

20/029/2013 16:29:10 - INFO - Flashing data

20/029/2013 16:29:13 - INFO - Processing system.sin

20/029/2013 16:29:13 - INFO - Checking header

20/029/2013 16:29:13 - INFO - Flashing data

20/032/2013 16:32:27 - INFO - Processing userdata.sin

20/032/2013 16:32:27 - INFO - Checking header

20/032/2013 16:32:27 - INFO - Flashing data

20/032/2013 16:32:51 - INFO - Processing elabel-c6603-row_201302281243_10.3.A.0.423.sin

20/032/2013 16:32:51 - INFO - Checking header

20/032/2013 16:32:51 - INFO - Flashing data

20/032/2013 16:32:53 - INFO - Flashing preset1.ta

20/032/2013 16:32:53 - INFO - Custumization reset is now based on UI choice

20/032/2013 16:32:53 - INFO - Ending flash session

20/032/2013 16:32:53 - INFO - Flashing finished.

20/032/2013 16:32:53 - INFO - Please unplug and start your phone

20/032/2013 16:32:53 - INFO - For flashtool, Unknown Sources and Debugging must be checked in phone settings

20/032/2013 16:32:54 - INFO - Device connected in flash mode