Sorry it took so long to get back on this. Apparently there was some changes on PayPal regarding the "cancel URL", hence the issue.
To fix this, you can edit machform/includes/post-functions.php file.
Search around line 3892 for this code:
$paypal_params['notify_url'] = $mf_settings['base_url'].'paypal_ipn.php';
exactly above that code, add this code:
$paypal_params['cancel_return'] = $mf_settings['base_url'].'view.php?id='.$form_id;
This should fix the issue.
We'll include this code into our next update.
Thanks for letting us know!