Also you appear to have mixed up UID with GID. The format in passwd is ...:UID:GID:... So you're talking about GID here. When you played around with GID, did you make sure a group with this GID did exist? I couldn't easily reproduce the issue with sessreg. Neither on Leap 42.1, nor with the sessreg binary of openSUSE 13.2. I suggest to debug this issue with gdb.