NextGEN Gallery – problems with Media Upload

After I changed my theme, now when I try to add images in a post the media gallery window comes up as a separate page, not an overlay as it did before. My gallery files are shown just fine, but clicking on “save changes” does nothing. I have tried deactivating the plug-in, deactivating other plug-ins for possible conflicts, reinstalling WP, everything and nothing works.

Appreciate any help on this as it’s a client’s live site.

Thanks!