@007bmorris, after I’ve removed the Google Analytics code CF7 works again.
I therefor think that the on_sent_ok GA function has an error.
One of my forms is on https://vlinderhof.com/contact/ and it includes this GA code
<!-- Google Analytics start -->
<script>
var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'UA-****1912-*']);
_gaq.push(['_trackPageview']);
(function() {
var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'https://www') + '.google-analytics.com/ga.js';
var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
})();
</script>
<!-- Google Analytics end -->