TokenMismatchException in compiled.php -- in most recent version of Safari 10.1

We have a Laravel 5.3 (Spark 2.04) app that we use to generate and embed (i-frame) registration/appointment/contact forms.

But in more recent versions of Safari 10.1 (and mobile Safari 10.3.2) we're getting

TokenMismatchException in compiled.php

But this only happens in Safari 10.1 as Chrome & Firefox work fine. And older versions of Safari 10.0.3 also work. I tested on my iPad that has iOS 9.3.5 and the form worked fine.

You view a sample form here: http://www.chrislionkids.com/contact-us/

Just click the Submit button and you'll see the error.

I tried adding the submission URI to exceptions array as per this SO post. http://stackoverflow.com/a/31910532/1337511

But still get the error. It's as if the javascript were blocked in Safari.