Here is the status:
1. I checked Open-E for errors: No errors at all.
2. I did log in to the ADS/DNS server through remote desktop: The time and date was correct.
3. I've made a new certificate.
- And I still get kicked off the NAS server.
BUT... one of my colleagues noticed that the domain-server's date and time maybe were incorrect.
I'll check it later and let you know if that caused the problem.