Troubleshooting "There was a problem connecting the video call." errors

Opening a new thread to help troubleshoot "There was a problem connecting the video call." errors with the new preview of video and audio calls (WebRTC).

If you're receiving this error after setting up WebRTC, please help answer the following questions:

1. Did you use Mattermost Docker preview for WebRTC? Or did you set up a Janus for WebRTC gateway? How about Coturn for STUN/TURN? (See documentation for more info).
2. What distribution and database is your Mattermost instance installed on?
3. Which browser/desktop app (and version) are the two users using? How about operating system?
4. Does the issue reproduce with more than one person?
5. Does the issue reproduce each time, if you try to call the same person 2-3 times? (sometimes we've had issues in the past where first try doesn't connect)

Thanks for testing the early preview feature!