OK. Here's what we know about the issue so far:
You know how there are three links at the top right: User page for your IP, talk page for your IP, and Log in / create user? The code that's throwing the exception appears to be trying to generate the link to the user page for your IP address. Instead of getting "User:x.x.x.x" like it's supposed to, the function receives just "User:". It then performs a series of checks, one of which is to identify links to namespaces with no parameters, like "User:" is, so that check fails and an exception is thrown.
What this says to me is that the software is either unable to obtain your IP address, or doesn't like what it gets and so discards it out of hand before it even reaches the stage of generating links for that toolbar.
When you go to
showmyip.com, is there anything odd about your IP? Are you, perhaps, using IPv6? (Not sure how complete MediaWiki's IPv6 support is, although I did see some references to it in the code so at least some is ther.e)