we've tried all solutions and it worked!
We've increased max_connections parameter and will monitor if we've increased it enough.
Selinux is enabled but in permissive mode so it shouldn't block anything, just log the events.
It was most likely due to a different lock file destinations in init file and nagios config. I'm unsure how that happened but maybe someone changed it earlier.
Anyway thank you very much for your assistance, we've definitely learned something new about nagios and nagiosxi!
