he server required a new Linux kernel (instead of the old 4.14.12, we're now running on 4.19.44) for security reasons. I also upgraded all software packages and made a new backup and all that.
Just regular maintenance.
The server is back up.
P.S. for anyone who is interested, the security reasons mentioned above is with regards to weaknesses in Intel processors nicknamed ZombieLoad, RIDL and Fallout and you can find something about it at https://cpu.fail.By Karn.