Google Tag Manager code blocking page rendering

We have Google Tag Manager tracking code scripts installed on this landing page that we created a while back:

https://www.evergladesholidaypark.com/tickets-lp/

These are the specific instructions that we followed exactly:

https://developers.google.com/tag-manager/quickstart

However, .. the page is for some reason not loading or displaying  correctly in any of these browsers that I've tested it with (on a Windows 10 PC):

Chrome
Edge
IE 11
Safari

The only browser that I've been able to get the page to load and display correctly in is Firefox Developer Edition v61.

Although you can view the underlying source code without any problems (and you'll see that there are many different javascript libraries being loaded), the only "invisible" stuff that's going on under the hood is PHP-based session management .. which I've already tried removing, .. though it didn't make any difference whatsover:

<?php 
  session_start();  
?>

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Why is this happening?  The only clue I came across with this message in the console when trying to load the site in Chrome -- although I'm having a hard time translating it:

VM32:2 A parser-blocking, cross site (i.e. different eTLD+1) script, https://ssl.google-analytics.com/ga_exp.js?utmxkey=71652754-1&utmx=&utmxx=&utmxtime=1526315626481, is invoked via document.write. The network request for this script MAY be blocked by the browser in this or a future page load due to poor network connectivity. If blocked in this page load, it will be confirmed in a subsequent console message. See https://www.chromestatus.com/feature/5718547946799104 for more details.

Any help would be appreciated.

Thanks,
- Yvan