Mattermost expired certificate error

We have MM 3.0 Version: 3.0.0 (3.0.2) installed in a VM on a Debian system. I updated to version 3 a bit over two weeks ago and everything seemed to work find.

Since yesterday, I get 304 errors in my Linux Mattermost Desktop 1.1.1. People in the group using apps on Windows and MacOS, as well as in different browsers and on android devices get Expired Certificate errors.

Here is a sample error message I get:

Your connection is not private

Attackers might be trying to steal your information from
(for example, passwords, messages or credit
cards). NET::ERR_CERT_DATE_INVALID

Would this have to do with the Mattermost installation or the server settings? Could the web-facing server have been tempered with?

Thanks