I'm getting countless messages that this site is offline and then offline, but every time I check it's online so I think something is going wrong on WPMU's end.
Uptime shows that the site was down due to the 503 Service Unavailable error. There are nothing at all in the server error log.
In the debug.log I see a lot of different notices and warnings but no Fatal Errors.
Also, I've noticed that "Notifications" for Uptime monitoring was disabled, but I can't remember that I did this myself.