Finally what worked:
- We connected the server via a ethernet cable directly with the PC.
- We set the ip of the PC to 192.168.1.111, so that it is in the same network as the Thecus server.
- At this point we could access the login dialog by opening 192.168.1.100 on a browser on the PC. We reset the Admin password by pressing "Reset" button for 10 secs just after powering Off and then On the server.
- Entered "admin" as username and password to login. But this won't work for multiple attempts. We pressed Ctrl+F5 on the browser on the PC to stop using the cached data on the browser. Only after this the login worked.