Gallery “Insert Into Post” not working

I have two blogs, both on 2.5.1 (I know, I know, I should upgrade to 2.6), both hosted on the same server. The gallery in blog 1 works flawlessly. I can add images to the gallery, and I can add them into the post using the “Insert Into Post” button.

Blog 2 also uploads correctly, and if I manually add “[gallery]” into the post I get the gallery. But whenever I use the “Insert Into Post” or “Insert Gallery Into Post” buttons, the webbrowser displays an empty gallery window and nothing happens. I can close the window using the X button, but that’s about it. The image isn’t added to the post.

Any ideas how this is working on one blog and not on the other? I tried Firefox 2, Firefox 3 and IE7. All have the same problem – works on one blog, not on the other.