Just finished a week with lots of these on the server console:
Process c:\lotus\Domino\STUserInfo.exe <4472/0x1178> has terminated abnormally
When this happens, no more users can log on. Already logged-in users have no problems though.
The only way to fix it seems to be to reboot the server.
We ended up with “downgrading” the Domino server to 8.5.1 SP4. No problems after this.