As per Microsoft:
This event indicates that the writer of the Volume Shadow Copy Service (VSS) of the Microsoft Exchange Replication Service did not exit the ongoing backup process when the VSS writer received a request to stop the backup. The VSS writer might not exit a backup process as requested because it did not recognize the backup set.
To resolve this error, do one or both of the following:
Run VSS backup again on the storage group on which backup was stopped. If the successive VSS backups fail with a backup-in-progress error, stop and restart the following services from the Services snap-in. This will clear the backup-in-progress state for the storage groups that are affected by this failure:
Microsoft Exchange Information Store service.
Microsoft Exchange Replication Service.
If you are using cluster continuous replication (CCR), stop the backup process that is running, restart the passive node of the cluster, and then try the backup again.