As you suspected, there was an old chat profile hanging around, attached to your account but referencing a user profile that no longer exists.
I deleted it; you should be able to log in fresh now.
For a bit over a year now, we've been automatically deleting chat profiles when the last associated parent profile is deleted; this should avoid such problems in the future.
If you suspect you may be encountering the same problem, please contact uscontact us and we'll fix it for you.