Password protected pages not working

Hi,
The plugin isn’t working for password protected pages. When I use my custom URL on the form for password protected content (sut-admin/?action=postpass), it gives the 404.

I have looked at the plugin code and can see in plugin.php on line 529 (conditional code below) that the postpass query is being blocked. I’ve tried removing the conditional and it works fine. I obviously don’t want to make this change to the plugin code but I don’t understand why it is being blocked intentionally as it seems to work across the website without it?

if ( ! ( isset( $_GET['action'] ) && $_GET['action'] === 'postpass' && isset( $_POST['post_password'] ) ) )

Thanks
Chris