Default [Q] Can't update CM11

Hi guys,

I'm new to Sony devices and rooting/flashing procedures.

I run CWM recovery and CM11 with the included Kernel 3.4.0-g14fe8eb, BL is unlocked.

As the title says: when I try to flash a cm11 rom, other than the one I'm currently on (2014-03-01), I end up in a bootloop (CM arrow doesn't stop spinning). When I try to restore a backup via recovery, I end up with a missing IMEI and thus a not working mobile radio. My procedure to fix this is: full wipe, flash the current nightly.
To me it doesn't make any sense that the IMEI is missing after a restore (at the time I created the backup everything was fine). What is the correct procedure to update to the next nightly?

Best regards,

Sony Xperia Z (C6603 Codename: Yuga)
LG Optimus HD LTE P936