I had this problem...and this is how I solvent it
:
1) I had account on wowhead and on wowinterface...same email...BUT DIFFERENT PASSWORD
2) So, I went to wowinterface, and change the password. The problems was the same.
3) So, again, I went to wowhead and change the password too. The same as in wowinterface.
4) And then I logged in smoothly
So, 3 different nicks, but the same password. I think the bridge between the different websites database first look on wowhead, then wowinterface, so u need the wowhead password.
Anyway, try setting the same pass everywhere