By default debug mode is enabled, this means that Quform will try to show PHP errors and exceptions inside your form, otherwise they would be silently hidden in an Ajax request.
Disable debug mode
To disable debug mode change the code at the top of process.php to define('QUFORM_DEBUG', false);.
Enable debug mode
To enable debug mode change the code at the top of process.php to define('QUFORM_DEBUG', true);.
Errors are still not showing
You should disable JavaScript (or comment out line 3 of js/scripts.js) and any PHP errors will be shown when submitting the form.
Errors are STILL not showing
In this case, you may need to check your PHP or Apache error logs.