Have you tried restarting XO (
sudo systemctl restart xo-server.service)?
If that doesn't resolve it, then it would probably be good to show us how you have configured your host(s) in XO (Settings > Servers).
VM backup only possible if VM is not running
Can you provide further details? What error(s) do you encounter?
VM backup also try to backup the storage HDDs -> so much capacity I don't have
What type of backup are you performing? You may want to read up on the following --
https://xen-orchestra.com/docs/delta_backups.html (see very bottom)