Connection issues with Edge and Chrome, may pertain to Firefox as well:
Symptoms: A small percentage of users have reported a problem when using Google Chrome or Microsoft Edge to join a Collaborate Ultra session or breakout group, causing them to be unable to join. The users will see a purple spinner that eventually resolves to the following error: "Uh-Oh. You can't connect. Try to rejoin"
In order to fix this, turn off Hardware Acceleration in Chrome or new Edge…. Settings… Advanced… System. Restart the browser.