Exchange 2013 backup fails with Backup Exec 2014

For the past week I've been having failures with my backup of Exchange. I get the errors:

V-79-57344-34070 - Snapshot Technology: Initialization failure on: "Microsoft Information Store". Snapshot technology used: Microsoft Volume Shadow Copy Service (VSS).
 

Snapshot technology error (0xE0008516): The database specified for the snapshot was not backed up because the database was not mounted.

All databases are mounted. The VSS list writes are all stable. Not sure what to do again.

Any suggestions?