I see this periodically in Filemaker Server 13.0v10 (and v9). Backup schedules will start, but will never end. The contents of the backups seem to be complete, that is, the folders seem to have all the database files and the Container folders seem to have all the container data. However, the schedule will not stop.
The only way to kill a backup schedule is to shut off Filemaker Server. I wish there was a way to kill an individual backup schedule.