It sounds as if there is something wrong with that person's user account and the way it is connecting in the code. 500 errors occur on the server side and is a general type error. Below is an article on 500 errors:
I would try recreating the users account and see if that helps since they can use an alternative account and get in.
You also may want to visit the forums for the particular program and see if they have run across that issue before.
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