You receive this error when you attempt to log into your personal space via the “Sign in” button and also when you attempt to join a meeting as a Guest.

The XMPP Server is guy who will authentication users, whether they be local domain users or guest users.

With Guest users. The web bridge sends the call bridge instructions to create a temp guest user account, the call bridge creates this account, then send the web bridge the username/password details.. The Web bridge, then will contact the XMPP Cluster to verify authentication for the newly created guest user.. As you can see below capturing packets on the EDGE device.. The port TCP 5222 is being blocked toward the XMPP Cluster.

Cisco Document references the below ports only.