SLES 11 Regigrstion Problem: wrong GUID

I recently had a problem that a technician could not register a SLES 11 system. The registration process ended with an error message from the Novell registration service: wrong GUID.

Running the registration from the command line...:

$ suse_register -a regcode-sles=..... -a email=.... -d 3

... showed that there was indeed a wrong GUID sent to the server. I grepped for the GUID string in /etc/* and found it in /etc/zypp/credentials.d/NCCcredentials. Deleting this file solved the problem, the registration worked as usual.