It might be a bug in django-social-auth. I've had a couple of people complain about it, but I can't reproduce it even if I create a new GitHub account. I've found bugs in DSA before, such as https://github.com/omab/django-social-auth/pull/697
I'll have to look into it more. Sorry about that. :(
"Sorry, but an error made this operation impossible."
Is this feature not enabled here, or is the site slashdotted?