I forgot.

That is what you get when you're working 13-14 hours per day (for the 6th week in a row now) University is to blame for my lack of sleep, but I'm still to blame for forgetting to announce this.
Please accept my apologies!

So, why was the web site not accessible for several hours (I think about 7 hours in hindsight)? Because the administrators of the new data center had badly configured the port mapping, thereby causing the web site to be inaccessible during that time.

Datacenter moves of course only occur very rarely. So expect this to be the first (since we're on the new host) and last major downtime for years to come.