I was unable to upgrade Proxmox. It turned out that internet access was lacking. More specific: the whole lan, with the exception of (the internal side of) the router was reachable. On closer inspection PBS, the backup server, had the same problem.
Reaching those servers from the router was not possible either. In the mean time the containers were accessible from the Internet, and backups were sent as usual.
After checking DNS, dynamic and static IP, bonds and bridges, ARP came in to view as the culprit.
arping gave timeouts on the IPs of these servers, while
arping on the hostname gave the right MAC but the wrong IP.
Cause of this all was the static ARP-entries that I set for a couple of machines, to have the MAC at hand for wake-on-lan.
After unchecking the check boxes, applying the changes and flushing the ARP table, everything started working immediately!