You need to go to System Preferences>MobileMe. If you are signed in there go to the Sync pane and turn off all syncing, then go to the iDisk pane and if iDisk syncing is running, stop it. Then go to the Account pane and sign out. This should stop any error messages.
If you had a MobileMe and did not migrate it to iCloud before 1 August then the entire contents have been deleted.
I have the same problem. I'm able to receive e-mails on my mobileme account but I can't send them (on my loptop). I can receive them and send them using the mobile.me (already migrated to icloud) site and though my iphone.
If mobileme was gone I don't think the other channels wouldn't work either, right?
And Roger, I wish I could try your solutions I am not able to try it becuase I'm not logged in and when I try I'm getting that error message.
What is weird, that same password works everywhere else.....
Does anyone else know what is going on?