Depending on the error code, there are multiple reasons to receive these message such as timeouts and VSS issues. Below are examples of a few errors.
Error: The guest OS has reported an error during quiescing. The error code was: 4 The error message was: Quiesce aborted.
Reason: Error code is 4 - VMBACKUP_REMOTE_ABORT. The backup was intentionally aborted or when when quiesced snapshot times out.
Error: The guest OS has reported an error during quiescing. The error code was: 5 The error message was: 'VssSyncStart' operation failed: The system cannot find the file specified. (0x80070002)
Error: The guest OS has reported an error during quiescing. The error code was: 5 The error message was: 'VssSyncStart' operation failed: The service cannot be started, either because it is disabled or because it has no enabled devices associated with it. (0x80070422)
Reason: Error code: is 5 - VMBACKUP_UNEXPECTED_ERROR.
Both errors above are thrown by Microsoft Service Control Manager needs more investigation with access to in-guest logs at the minimum.