I am also experiencing this problem, and although taking the steps above seems to have fixed the login issue, I am not receiving any mail from these accounts (which, from what I've read in other threads, is also happening to other users).
Any idea what to do next? I am sort of at a loss.
[EDIT]
I should also note that although accessing mail through my desktop email client works ("works" as in "authenticates", but i have yet to receive mail through these accounts), webmail is still broken. I use the same account and password but webmail will try for a long time and eventually return an error saying that either the username or password is incorrect. Perhaps a problem with DNS or an apache conf?
[/EDIT]