Webserver was offline this night
When I woke up this morning I realized that the webserver was not reacting anymore. In general the machine was rather unresponsive and had a load of >240.
The issue began after 3:17 CET according to the last log entries. You can see that the disk utilization graphs get a peak at that time: https://nerdica.net/photos/ij/image/57847342405c29eaa39b252403735375
Even rebooting via SSH was not possible anymore. So I had to kill the virtual machine and restart it.
Now everything should work again.