At the risk of stating the obvious, somehow, a file got created with same name/location as Lightroom is trying to create a directory.
I dunno why/how that happened, but I recommend 2 courses of action:
1. See if you can figure out which file/directory (e.g. look in system event log). If file is expendable, deleting it may fix your problem. If not, then Lr is trying to create a directory in the wrong place. It could be that renaming your preferences file will fix the problem (granted you'll have to re-enter preferences..).
2. Report bug to Adobe: