What I can't tell you is what set that particular combination on your VM in the first place. Does it ring a bell?
I have no Idea. I had it on "Protect from accidental shutdown" but turned that off again, later. Doing this again (on, off) helped, as you said.
Thank you so much!
@poddingue the response is "VM_REQUIRES_SR". The VMs are storing their vdisk on a local NVME. This is crucial for this hypervisor as I do not want it to depend on network storage. Can I work around this error in such a scenario, i.e. is a Rolling Pool Update possible under these circumstances?
Does the Smart Reboot require evacuating the VMs? Technically it shouldn't as in XO there will be a warning that can be manually overridden. WHy do I get the warning in case of a Smart Reboot?
@olivierlambert Thanks that worked. Kind of obvious approach, still didn't try that 
@olivierlambert I have seen that part of the documentation and it did not contain a solution for me. I also talked to the community on the Teleport Slack and some staff member tried to solve it, but was unable to. So I hope the Teleport discussion forum on github will lead to some solution, so for not.
So lets please keep this post open as someone here who is using Teleport might see it and answer, while that person may or may not follow the Teleport discussion on github.