For a moment I was really happy but it seems that two new problems appear after I solve one. I managed to flash a matching kernel, could connect to my sdcard via pc and put the custom roms on it. Tried to flash the kernel with the boot.img in the Custom Rom's archive, which I have done yesterday and which worked just like a charm, and now it doesnt.
Flashtool shows me this error:
26/052/2013 21:52:16 - INFO - Device connected in flash mode
26/052/2013 21:52:27 - INFO - Launching Fastboot Toolbox 1.0 by DooMLoRD
26/052/2013 21:52:27 - INFO - Device Status: NOT FOUND
26/052/2013 21:52:29 - ERROR - This action can only be done in fastboot mode
26/052/2013 21:52:34 - INFO - Finished Fastboot Toolbox
26/052/2013 21:52:55 - INFO - Flash canceled
26/053/2013 21:53:00 - INFO - Device disconnected
Seriously?! What the hell is going on, I feel like the biggest idiot ever, right now. Other than that, is it alright if the custom rom's a rar-file or does it really has to be a zip-file.