Sometimes the incremental backups of Exchange may end up with a warning, and in the message log there will be following string:
ESEBackup: BackupSetup failed error c8000230:

That usually means that the incremental backup misses some parts of the databases, which should be included into the full backup that incremental rely on.
In order to fix this issue run the Full Exchange (file level) backup. Once it will end up successfully, the next coming incremental backups should be good