This problem comes up when the nonce has expired, this often happens due to caching.
An easy way to reproduce the issue is to add the following to functions.php (and make sure the page isn’t cached and you’re not logged in):
add_filter( ‘nonce_life’, function () { return 15; } );
Then wait 15 seconds and try and submit the form and then check the console log.
From what I understand, the plugin “tries” to fix the issue by requesting a new nonce – but you can’t request nonces unless you’re authenticated so this doesn’t work.
