Found the following error in my Miscellaneous Events log after a crash the other night. It occured after a Notes Client could not access the server, but server access could still be made through HTTP and SMTP.
ERROR MESSAGE: Notes error: Insufficient memory - server executive pool is full.
Turns out it's a problem with Domino 8.5. There are 2 fixes, one short term, one long term.
Short term, reboot the server. That'll give you around 2-3 days before the server process has memory issues.
Long term, upgrade to Domino 8.5 FP 1. It solves the issue.
There's also a Notes.ini variable you can add short term.