Nagios Support Forum

Thanks for the data, everything looks good so far.
If that service is still not sending notifications, disable the non-working one, create a new service entry and see if the new one works.
It could be a corrupt entry causing the issue.
Let us know if this works or not.

Good news and bad news .. Mostly good though.

Bad news:
It didn't work. I created some new simple checks and some new Nagios XI users and the problem persisted. Also, this notification problem was actually happening on all checks including the new ones. Only one person (or in some cases nobody) would receive the alert.

Good news:
I disabled memcached and the problem went away.

My apologies, I only just remembered having made that change a couple weeks ago, back when I was trying to improve the IO situation on our Nagios box (just a few days prior to updating Nagios). I had moved all our performance data to separate disk, and .. I enabled memcached by following the instructions at: https://support.nagios.com/kb/article.php?id=194.

I guess there's some weird unexpected cache behavior going on with memcached on

That is probably it. There were a lot of changes done to XI for security reasons and also changes to MYSQL that may not work with memcached.
Or the memcache servers need to be restarted in case one on them locked something up.
Glad that it is working for you now.