I believe I have the solution to this issue and am sharing my findings here for anyone else (like robdowns) who ran into this problem.
I worked quite a bit with the Authorize.net support engineers and this is what they suggested:
1) The structure of the zip files being used need to exactly match the ones attached in the email. If they don't match, then some things may not work correctly.
2) If the token of the page is no longer valid, then that could cause an issue.
3) The domain of the url for IframeCommunicator.html has to exactly match the domain of the page that launches the popup.
4) Check the server logs to make sure that the url for IframeCommunicator.html is getting hit.
While none of this helped me directly, point #4 made me realize that indeed my contentx/IframeCommunicator.html never got hit, which had to be the problem. After poking around I looked at my code that makes the XML request called getHostedProfilePageRequest, specifically the url I set for the hostedProfileIFrameCommunicatorUrl setting. I had originially set it to the page that calls the popup, which seems to be incorrect. I tried changing it to the url directly to my contentx/IframeCommunicator.html... and now the pop up closes!
So the issue was an incorrectly defined hostedProfileIFrameCommunicatorUrl in the XML call. I'm not sure if my new setting is the correctly solution to the error or if it causes any other side effects, but it seems to be working just fine so far. I want to note that the documentation for this is poorly lacking and very vague - the best I found was in the XML support guide on page 17. I recommend the support team makes it more clear exactely where the url should point (also the spelling error is still present, so don't get confused by that).
SOLUTION RECAP: Make sure your value for the hostedProfileIFrameCommunicatorUrl setting in theXML getHostedProfilePageRequest points directly to the contentx/IframeCommunicator.html. Hope that helps!
