Facebook login not working

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Hi, I am trying to integrate FB login into my Askbot website. I configured everything in my website, but once I click on facebook's image I am redirected to a FB page that says that there was an error.

Looking at the url, I found this:

redirect_uri=%2Faccount%2Fsignin%2Fcomplete-oauth2%2F&

if I change it to

redirect_uri=MYDOMAIN%2Faccount%2Fsignin%2Fcomplete-oauth2%2F& it works.

Did I configure FB wrong or is it an issue?

Thanks! Oscar