Since yesterday evening, my site has only been available very sporadically. As of noon today, the site was almost completely offline. In most cases only the error code HTTP 500 was delivered. The entire website including all subpages was affected. Unfortunately, I was only able to take care of the problem now.
The culprit was a WordPress plugin that was supposed to cache the pages for faster access. The plugin has been in use for quite a long time and had previously performed well. Apparently, however, there was now a malfunction. Since the plugin has been deactivated and the cache deleted, the site is working as usual again and the first comments are also starting to trickle in again.
I will look for an alternative for the plugin and test it extensively beforehand. I think I will also think about an "emergency" system to be able to intercept such longer outages in the future.
By the way, this was the second longer downtime in the 8 years that this site has existed, the first outage was a few years ago and was a routing problem in the data center at my old hoster. So now a WordPress plugin for a change.
Incidentally, that's how quickly the availability statistics of 99.9 % are gone 
An dieser Stelle von meiner Seite ein dickes Lob für deine Seite, Recherchen, Dokus, Anleitungen.
Auch komplexere Themen sind sehr gut und verständlich beschrieben.
Lieber Frank,
Absolut kein Problem, solche marginal kleine Unterbrüche passieren halt mal und gehören zu einem IT-Betrieb.
Bei dieser Gelegenheit: herzliche Gratulation zu Ihrem hervorragenden Webauftritt, inhaltlich und funktional absolute Spitze mit Referenz-Charakter. Ich wünsche Ihnen nur das Beste weiterhin und immer gute Erfolge.
Herzliche Grüsse aus Bern/Schweiz.