Hi Morten,
This sound a lot like a problem I have with Allied Telesis iMG616 devices. I think it is a bug in the client.
For some reason the client is reusing some part of the credentials after a reboot and OpenSER refuses to register the client because it thinks it is a replay attack. Sorry for the vague description but I cannot remember the details.
Yep - that's about the same conclusion I've reached.
Regards, Lars