Delf on IRC found this, at first I thought he was imagining it. :P
If you click a mumble:// URL from outside the client, or in the chat log of the client, it functions fine. If a user has a mumble:// URL in their user comment, and it does not match the connection database (which I believe will prevent the username dialog from appearing) the dialog pops up, but cannot be edited.
I've confirmed that both cases MainWindow::openURL() is used, and the offending line is user = QInputDialog::getText(...) (line 723 in HEAD at time of writing). It is executed in both cases and fails in one, which makes me wonder if it's a QT bug.
Hope that helps. :D