Now I was able to Flash cm-11-20140618-NIGHTLY-v500.zip, although i edited the update script and removing the first line where it verifies the model. So it flashed the ROM correctly, but upon restarting, I ended up with the 'official' "LG Logo - Security Error".
So I did a restart via Power+Vol- into All three buttons. The Factory Reset screen comes up, when I hit Vol+ twice I gets me back into TWRP, but on the way there it also shows up the
Secure booting Error!
Cause : Device Unlock!, so Boot Success!!!
.... and now?