Found this in my inbox today:
“Thanks to help from the IBM development team and support the issue some customers have been experiencing when running custom server tasks and Extension Manager on Domino 8.5.1. has been resolved.
The root cause of this issue was that IBM has introduced some I/O performance improvements for logging with 8.5.1, which unfortunately had the sideeffect that servers were haulting when running custom server tasks … .
But the good news is that IBM introduced a way of turning this new logging off and reverting to the way logging was done in 8.5.0.
So all you have to do is put in an extra line in the notes.ini of the server.
After that everything works again.This has already been validated. The only adverse effect of this change is that logging will not be faster than it was in 8.5.0. We expect the issue to be fully resolved in the next service release of Domino.”