We have upgraded from commit f2188 to cb6cf and have 3 hosts. With none of these hosts backups worked anymore after doing the upgrade. When reverting back to f2188 all works again.
We also deleted orphaned VDIs and let the garbage collector do its job but it did not helped.
Hosts/Errors :
1.) XCP-ng 8.2.1 : "VDI must be free or attached to exactly one VM"
2.) XCP-ng 8.2.1 (with latest updates): "VDI must be free or attached to exactly one VM"
3.) XenServer 7.1.0 : "MESSAGE_METHOD_UNKOWN(VDI.get_cbt_enabled)
With the Xenserver 7.1.0 CBT is not enabled (and can not be enabled).