I was having the same issue, and after some debugging found that the following GTM code below was causing the issue. I tried changing async to false and the error still occurs. This often happens after clearing your cache. Not sure if that helps point someone or the developers in the correct direction?
<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','GTM-XXXX');</script>
If others could verify the above that would also be helpful in determining the cause.
