Let’s Encrypt certificates are broken on Mattermost servers. We are working on a fix, and will deploy a hotfix release, v4.6.1, this week.
Progress can be tracked through this ticket: https://mattermost.atlassian.net/browse/ABC-198
A workaround is to terminate SSL at the proxy, or set up let’s encrypt manually directly on the Mattermost server via the “TLSCertFile” and “TLSKeyFile” config.json settings.
For any questions or concerns, let us know by replying to this forum post.