Just to say that I have more or less the same thing. In the WebAdmin (advanced->configuration->uPnP server) I see only a login screen with a username password prompt. When I enter this, nothing happens. When I do sudo /etc/init.d/mediatomb start everything is ok, but after two minutes of working with the tool the message "MediaTomb cannot be reached" appears. Reload of the page gives me the username/password prompt again. Starting the server from a terminal restores it again, etc ...
The strange behaviour started after loading many megabytes of pictures, but I doubt there is a connection.
I work with the final 8.10 release.