YaBB support had me shorten the forum name, and rebuild the userlist and userinfo files and they rebuilt fine. They say if those files had been trashed they wouldn't have rebuilt and I looked at them and they appear fine. According to them this indicates this is something wonky with the host. They say, and I agree, that if everything has been working fine and all of a sudden doesn't work, and you have made no changes or mods to the forum, then it has to be a host issue.
I noticed, after a lot of jumping around, that my "clock" in the upper left displayed different times and different usernames depending on the "board" I am in. I also notice that when I get into a board if I do a <shift><reload> then my name comes back along with the correct time. It's almost like I am being served up a mix of cached and live data. I have asked Suraklyn to query the hosting company to see if they have made any server changes.