That cPanel username is linked to the Forum username
shop123
1. You should not have multiple Forum accounts.
2. If you accidentally created a second Forum account, please use the Forum account that is linked to the cPanel account. It will keep you from being suspended for inactivity and it makes it easier for us to help you.
500 errors are usually caused by one of two things.
a. scripts that do not have the proper permissions or fail miserably
b. problems with .htaccess
Since the error happens when you try to access the main site, could be a problem with your default index page. Is it a script? index.cgi or index.aspx?
Otherwise, please post the contents of your .htaccess file. That way, we might find the problem.