I have gone through your problem and based on the information you have provided I think it is a permissions related issue on your file. It would be one of the causes of a 500 Internal Server Error which you are facing.
To make sure the problem and it seems you do not have more detailed information related to the error you need to navigate to your directory in the Terminal and simply run the below command:
After running above command if you come across the limited permissions for example.
-rw-------@ written against your file, then it confirms your problem is the limited permissions.
Now solution on it is very simple you need to run
chmod 644 on your problem files or
chmod 755 on your directories.