Hi Beki,
I may have spotted what is wrong: it might be that the popup report is blocked by your browser, please see if the following helps.
How to allow popups from a specific site: for Internet Explorer (IE), or FireFox.
If it does not, and if you are using IE, you may want to add "http://eppi.ioe.ac.uk" to your list of "trusted sites" (instructions here).
Otherwise, I'm sorry but I might need more information to isolate this issue. I've tried to run comparisons in your review and couldn't replicate this. Are you accessing the comparison features from the "coding record" tab in the "document details" window or from the "collaborate" tab (main screen)? In any case, since this appears to be difficult to replicate, I will need to know exactly how you are able to trigger the error: this includes the specific document you are looking at (if applicable) and the code/code set you are working with.
If this does not help, it is possible that we won't be able to fix your problem during the week end: for security reasons, there are lots of things we can't change from home.
Please do let us know if any of the above helped.
Best wishes,
Sergio