Been searching around for this one for a long time, but it doesn't
look like anyone has had the error at the exact same time I have.

I was getting the "The messaging interface has returned an unknown
error" error in outlook 2003 attached to Exchange.

Blew away the profile, created a new one, and it WORKED. However
after the new .ost file was created when setting up his new prof, the
issue returned.

Does this point to some kind of corrupt data in one of his folders?