Intermittent connection issues to server based on browser/app used

Connection to self-hosted Mattermost server is intermittent and success varies based on platform, appears to be related to SSL certificates.

Steps to reproduce
I’m running a Mattermost 6.1 server on a DigitalOcean droplet. I set it up using a Let’s Encrypt SSL certificate (not using TLS) and have it running through our domain via Google Domains (set up A record to direct to IP).

Several times per day, seemingly randomly, I am unable to connect to the Mattermost server. Confirmed affected platforms:

  • Mattermost Android app
  • Firefox browser on Android
  • Desktop app on Mac OS

These platforms seem to always connect without issue:

  • Firefox, Chrome on desktop (via website)
  • Chrome on Android

The issue only occurs when trying to connect through our domain When connecting directly via IP, it gives me a warning about connection security but allows me to connect.

Expected behavior
Up until recently, all connections were stable and secure. I changed DNS settings recently but reverted them back and it’s been > 24 hours so all changes should be propagated and stable. When connecting successfully, all browsers confirm a secure connection (lock symbol) Verified by Let’s Encrypt.

Observed behavior
When I cannot connect, I get the following errors:

  • Mobile app: “Cannot connect to the server. Please check your server URL and internet connection.” (ran through troubleshooting tips on website)
  • Desktop app: ERR_NAME_NOT_RESOLVED (-105)
  • Firefox Android: Unable to connect
    Typically 10-30 minutes after the connection issues starts, it resolves and I am able to connect again… until the next time.

Hoping someone has seen this before! I have been scouring forums and trying things for a few days now with no success.