When either clicking the "Join Mumble" link, or the same getting triggered because mumble is mandatory in a particular lobby, after all slots have been assigned, your name gets prefixed with your class - nothing new. However, the format "CLASS: Playername" collides with the Mumble URL scheme, which interprets a colon as seperator between username and password.
This means, if the first part of the Mumble link called is for instance "HEAVY: Little Hoovy@[...]" the client would, unless the colon is masked, enter the server as "HEAVY" (which often collides with existing players, cf. this post), and the password supplied would be " Little Hoovy". Since there is no password of which i knew, that part just gets ignored.
You can read more about this at the Mumble wiki: http://wiki.mumble.i...wiki/Mumble_URL
Solutions which come to my mind would be either using the colon's HTML entity, if possible (%3A in URLs, this post might be related), or simply leaving it out. I think "HEAVY Little Hoovy" would be equally well readable in the overlay.
Best regards