Ordinary 500 error that some HTTPD server error - you may check it in your site error log.
Also about 500 error you may ask from your hoster - he may see more errors in self logs
next you may try check that post(i not know what version you use)
but you will need check that in file
if all before not help, please send to us admin and FTP access to your site and we will try check where error.
Your account details you may send with help Ordasoft contact form