There was a server outage in the middle of the night, sadly I have been sleeping.
I used the downtime to install some seucrity updates and improve the firewall and network configuration, which will reduce future downtimes. I also made sure that the newest drivers for the network interface are installed, because the problem seems to be a network interface getting lost / stuck.
I also fixed a problem on one of our firewalls, so for an unexpected downtime a lot things improved, which I hope increase the stability even further. Servers will be back in a few minutes.