Thank you @reinob your answer solved it (see below)
I have indeed not done a zpool upgrade, for a number of reasons:
Sofar I have not upgrade any of the pools, the message has been there for quite some time
Every description of zpool upgrade...
Thanks for the reply, but I already tried that from the web interface (and now also from the command line), it yields the "same" error:
timeout: no zvol device link for 'vm-111-cloudinit' found after 10 sec.
Today I updated my 3 servers and rebooted. Everything seems to work fine except for one VM: my printserver
It no longer works because the VM (111) running cups will not startup:
TASK ERROR: timeout: no zvol device link for 'vm-111-cloudinit'...