If you are getting HTTP ERROR 500, this means that there is some error thrown in the back-end which is causing Perfex CRM to stop working.
In order to easy to determine the issue, the best is to turn debug mode as explained here
If the error messages are not clear enough to help you to resolve the issue, you can open a support ticket at https://customer.portiawebsolutions.com/, we will be happy to guide you further and help you resolve the issue in a timely manner.