A 500 error is a very general error message and a common topic of discussion here. Since it sounds like you're on a Plesk plan, have you tried some of the suggestions already posted on 500 - Internal Error Message.
Another thing you may want to try is to enable detailed error reporting or you may want to search for some of the other 500 error discussions we've had started on the community as these are caused for various reasons. Let us know if you manage to get this worked out.
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