Since this week we've had three occurrences at our office of a dialog popping up saying that secure communication for the office apps was not trusted because the vertificate for outlook.office365.com was revoked. Its that one specific domain.
When researching the problem, most of the fixes found we for issues on the upstream firewall, in our case an MX84, but the solutions were for Sonicwall and Cisco ASAs. On the Sonicwalls you had to whitelist a pair of Digicert URLs, and other actions for the ASA (sorry don't have that link right now). And a majority of the incidents I read about reported that apparently fixed the problem. Today we had a customer behind a Sonicwall experience the dialog, so the fix for that is in place, and we'll have to see if it repeats.
I can't find similar on the Meraki MX. I have an open ticket but they've stated there is nothing on the MX that would do the equivalent, and the problem has to be further downstream at the per PC level or any intermediate firewall (none at our location).
So far we can't find anything at the host level; the problem is very sporadic so we don't know if a test system with the windows firewall turned off not getting the dialog (so far) is relevant.
Has anyone seen this before? Got a fix?
My thought is also that the PC is an issue. Try downloading/running the support tool on the affected PC.
+1 to @SoCalRacer .
A common cause is the date/time on the machine. Are you sure the machines date and time was correct? It wasn't 10 minutes into the future or anything like that?
This wont be an MX issue.
There is another possibility. That is it malicious, and someome is deliberately trying to intercept the traffic.