If it provides any useful background I'm getting the same thing, but it was an account that I migrated from mobileme.
So after I did that there was a new 'iCloud' inbox and the old mobileme inbox, I deleted that one because it wasn't working after moving (through outlook either, so I deleted it there too). The inbox that was left was the iCloud one, which is getting that error. I also get the error that I can't use it for iCloud backups, so I don't know if it has something to do with it being moved from MobileMe?
Also I have a different apple id on the phone that was there after upgrading, a yahoo.com account, that is what I have on my itunes and seems to be working for that, I can see purchased music, tv shows etc.. so not sure if the two account could be causing some kind of conflict? It'd be great if I could use two account so the family can access music (well, ok the wife)..
Anyway hope that provides some background info, not sure if your situation is similar.