OK. Yes my doctype was missing and this was a big problem. I added this: <!DOCTYPE html> becuase it's HTML 5. And guess what? The ajax popup is now working properly, just like you suspected. I did run the W3C validator and I did get several errors. Most were small and unrelated. The larger issues I completely deleted temporarily from the template to see if it would fix (after a hard refresh) and none of them solved the issue.
So the doctype fixed the AJAX popup when an event is clicked on however; the previous, next, and today functions on top of the calender return a blank page. This might help, when I view the source of the blank page (after clicking the previous, next, and today functions) all I see is this in the source code:
<script id=__ie_ondomload defer=true src=//:></script>
It only shows in IE where the problem persists. In Firefox all is well.
What the heck is that? If my suspicions are right, this is the problem. The only issue is I don't know how to fix it and where the problem is located. Any ideas?