That was it :)
Um ... any ideas on where the problem might lie?
I'm a bit stumped on this one. If the plugin were using the sub-domain and the main part of the site was using it's own domain, I'd have put it down to a cross-domain AJAX request not working. But it's using the mapped domain for the AJAX call anyway, so I don't think can be it.
Or perhaps we should just call on the Rennicks aka "multi-site gurus" for assistance?
This isn't a big deal if I can't use the plugin. I was mostly just installing it since you had made it so wanted to make use of your handywork, but I'd be quite keen to at least figure out what is causing this. It seems like it should work.
At least it can be narrowed down to the domain mapping causing the issue now.