After digging through some log files do I have a fairly good understanding of what happened. But there are still a two huge question marks that I will try to straighten out if possible. However, the most likely explanation so far is that someone made "a change so small that they didn't need to to inform the customer" combined with a security setting in the database. A small reboot and...
I still have a lot to do to be compliant with that minor change, but it should not impact on the availability of the forums anymore.
Also, thanks to everyone who mailed me about the problem. As Owen mentioned on FBR, I am in the middle of preparing for a move to a new apartment. So my available Internet time are quite reduced at the moment.