Since Bitbucket Server 5.4.0, where the JIRA Guaranteed Delivery feature was implemented, it is possible for exhaustion in the ExecutorService threadpool to propagate to the event threadpool, possibly causing the event queue to overflow.
A system impacted by this would see all its event threads blocked with this stack:
java.lang.Thread.State: WAITING (parking)
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x00000004492e9ac8> (a java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask)