We have a virtual domain controller (Windows Server 2012) that is backed up by Veeam every evening. Occasionally we see the error "ErrorL VSSControl: Failed to prepare guest for freeze", and the only solution to this is to reboot the domain controller and run the backup again - after which the backup of the domain controller completes successfully.
When rebooting the domain controller we have to be careful to check the boot options in msconfig, because we often find that the Server is set to Safe boot in Active Directory repair mode. Unchecking Safe boot and changing the startup selection from selective startup to normal startup does not remove the safe boot option and we have to run bcdedit /deletevalue safeboot to remove the flag. Once this is done we can safely reboot the Server and it boots up as normal.
We can't figure out however why it occasionally does this, and we are wondering if it is something to do with the Veeam backup. I realise that the Server will boot into DSRM mode if a restore is performed, however we have not performed a restore of the domain controller and indeed never have to date, so I'm unsure why this keeps happening. It doesn't appear to happen after every reboot, but something is clearly putting into DSRM mode.