Try caching the user name for a new agent earlier on in the process of establishing a connection, to see if this helps with "Unknown UserUMMTGUN" issues.

The UMMTGUN form of Unknown User seems to appear because a viewer sometimes sends a UUIDNameRequest UDP request that fails to find a binding.
However, in theory the incoming agent should have made that binding before any such request is triggered.
So moving this binding to an earlier point in the process to see if this makes a difference.
Unknown user name is also updated to UserUMMTGUN2 - if you see the old name then you need to clear your viewer cache.
This relates to http://opensimulator.org/mantis/view.php?id=6625