If you get a redirect_mismatch the specified redirect_uri does not match the redirect URI registered for your application. Make sure it does. I think even the presence of a trailing / (or the lack thereof) matters; as it should.
AFAIK there is no other condition resulting in a redirect_mismatch, at least I never encountered one.
If the redirect_uri really does match, could you post the two URLs (with all sensitive data removed of course)?