TTS is enabled and configured in PBA. One of the ticket statuses (e.g.
Stalled) is disabled in PBA Home > System > Settings > Helpdesk > Statuses. As a result there are may failed tasks like below:
Event Handler TTS Notify Tickets TTS RemindDueTickets Status Failed Comments Execution Failed: StalledTicket Status is undefined.
The issue is caused by the bug with id #PBA-60009 ("TTS Notify Tickets event fails if some ticket status is disabled").