After trying every possible option (direct connect, different browsers, accessing from different systems like Windows and Linux) I decided to take an older version of the software, which had already proven to work in the past and installed that from CD.

Now I can access the WebGUI again and manage the NAS, but I'm back to version 4.21.NE00000000.2745

Still baffeled about the cause of all the problems. Hopefully a solution will come in a future update.