Do you know if the page that is working correctly is on the same server? I'm curious what happens if you were to switch these working and non working versions. On the site that has this working make a backup of the page and the Universal Email folder, then add in a page that has just the UE server behavior on it from the page that is not working, also make sure to use the non working Universal Email folder on this site. Test this out to see if the non working one begins to work.
You can also test this in reverse by backing up all of the non working sites parts and replacing them with the working versions. When you test this out it should help to determine if the problem is one that is specific to the site, or just the pages that are involved. Along with this info please also post back with a copy of the email folder from the site that is working.