If the incremental backup is failing with log files missing or mismatched sequence numbers then you will not be able to properly restore from the incremental. as replaying the logs during the restore process will not work (please note these are transaction logs that are key to database operation not just activity logs)
Typcally errors of this type are caused by the activity of another process or application performing VSS operations against the databases (or the volumes that contain the database) in between the Backup Exec operations and you need to track down the cause to eliminate it.
Oh and I wouild strongly recommend getting off BE 2012 - it is out of support and had a few conceptual design isues that did not work well in practice and were addressed in later versions (make sure you check the compatibility lists for later versions before going to a newer version though)
