The reason you are seeing "Internal Server Errors" everywhere is that JohnCMS tries to send emails at the very end of almost every action (registration, posting, or even some downloads).
Since your hosting blocks the proc_open function, the script crashes at the final step. It might even process your data correctly, but it fails to show the page because the mailer cannot start.
To fix this:
- Set up SMTP in mail.local.php. This stops the script from using the blocked server function and sends mail through an external provider instead.
- Enable Cron in constants.php. This moves the email sending to the background, so the user doesn't have to wait for the mailer to finish, and the page won't crash if the mailer has issues.
Once these two changes are made, the "Internal Server Errors" should disappear because the script will stop calling the forbidden function.