The recently published PHP vulnerabilities also affected this host (as, unfortunately, wordpress is written in PHP). I did not update the debian system on TheTheoreticalDiver for far too long. The price was that it did not have the required updates for PHP available anymore and thus I had to update the Debian first. Which resulted in an unresponsive machine. Hurray.
I had to reinstall everything from scratch. But luckily, I had made sure I had a backup of this blog. Unfortunately, UpdraftPlus, the PlugIn handling these backups turned out to be moody upon restore. So I had to basically do the restoration manually. But it seems, now, more or less everything is in place again.