So I got the confirmation from the provider that we were, in fact hit by a DDOS attack on Tuesday. Despite all the (more or less hilarious) attempts to be hacked over the last years, this was the first time something like this happened. Since I had no idea what was going on and since it happened 4:00 am in my time, it took me quite a while until I woke up and was able to call the provider regarding the issue after I tried several things to fix it myself since I had to go to the office and was quite busy there.
The provider realized the issue and started filtering the attack successfully. Here is a graph of the network traffic during that time:
So you can see it was quite heavy compared to what normally happens on the server. The daily bigger spikes that you can see early in the morning are by the way the off-site backups that are done every day.