The problem is that I can't reproduce the error -- the dropdowns work fine for me. This makes debugging pretty much impossible on my end alone. So, to try to find the cause of the error, I created two simple HTML pages, one using a standard HTML <select> dropdown, and the other using an ActiveWidgets dropdown. I sent these two pages to the user, and he reported that both of them worked fine.
I then uploaded the two files to our download page, and instructed him to open the pages directly from there (which would mean accessing them through an https domain). This time, he reported that the page using the standard HTML dropdown worked, but the one using the Combo widget produced the error.
Therefore, we came to the conclusion that the problem only occurred when using the ActiveWidgets Combo through an https domain. Something in this user's network settings, firewall, or whatever, must be preventing AW from working over https.
Unfortunately, that still doesn't provide us with a solution. I've already searched through the forums, but I can't find an answer. The test pages reference the AW files through relative URLs, and I don't think there's any cross-domain scripting involved. We also tried enabling the "Access data source across domains" option (as suggested in another thread), but that didn't help either. At this point, I'm all out of ideas; I'm hoping that someone here can shed some light on this issue.