First, I tried the SIM in my phone - working. Then I tried my SIM in her phone - working (so no SIM/Net-Lock). Before flashing CM 10.2 I made a backup of the stock JB 4.1.2 using CWM Recovery. So I restored that backup and tried it again. The SIM was working.
I couldn't find anything related to that problem in the forums so I tried to diff the system folder of both backups (stock 4.1.2 against CM 10.2 4.3.1) but there are way too many different files to spot the problematic one(s). I also had a look at the tee folder in system/lib but the files in there are exactly the same.
Is there anything I can do to find out, why it is working with 4.1.2 and not with 4.3.1? I'm also puzzled why the old SIM is working fine but the new one isn't!
P.S.: I couldn't post in the development forums because I just registered. That's why I opened the thread.