Phone Bootloader Relocked, no longer unlockable

Hello Guys,
i've got a Problem with my C6903.
Yesterday i unlocked the Bootloader of the Z1. (No TA Backup was made ). Today i Relocked the Bootloader with Flashtool, because there where no usable Custom Roms available. But then i thought why not try some different Kernels?! And now here's the Problem:
I can't Re-unlock the Bootloader. The Fastboot Command says when the first Time entered: Command did not succed. The second Time i enter it it say Device already rooted. Then i'm able to boot from a custom Kernel, boot to CWM and install AOSP Rom. But after Restart the Screen goes Blank (like Locked Bootloader). Flashing a Rom/Kernel through Fastboot just says Command not allowed. In Flashtool, i'm able to Unlock/Relock the Bootloader, well the Tool still says after Unlocking Rootable, not Rooted. But also after unlocking in Flashtool the Problems resist. Tried Flashing Stock Rom with Flashtool, with PC Companion(Repair) but no succes. Service Menu still means Bootloader Unlock allowed: Yes.

Would be nice if you Guys could help me out