To be honest, this doesn’t sound like an issue specific to osTicket Awesome. But I am willing to try to help you narrow down the cause of the problem.
First, try clearing your browser cache and try again.
If that doesn’t help, try the Email Access Link function through the osTicket core interface (hopefully you kept a working copy of your original osTicket installation). Let me know if this works or not.
HTTP Error 500 is a very generic message that is displayed on the web page. The actual error message is recorded in the error logs, usually the Apache error logs. Have a look for the relevant message that is being recorded at the time that the Email Access Link form is submitted. Feel free to post the error here.