The first thing you could try would be to delete each of the mobileme accounts and then re-add them. Since mobileme is based on IMAP, you won’t be loosing anything. If that does not resolve the problem, the second thing to try would be to navigate down to the file path pointed out in alert window and delete the corresponding folders, then re-add the accounts once again. it could be a bit messy. Let me know if you need any assistance.