I have a website running 2.7, and this problem has persisted since it was on 2.2 or earlier, I think.
I’m currently using two plugins that insert buttons on the Visual Editor, within Post/Page write interfaces: NextGen Gallery and CForms. When you click either, a lightbox popup window (or thickbox or modal, however it’s called) should appear on top of the page, the same way that the image uploader does.
However, none of them loads the plugins setting page, or anything, but an error page from my site.
I figure this must me a servers error, probably due to the fact my site is running as a subdomain. Maybe I should look at the code each button executes to open thath popup and call the appropiate URL, but I don’t quite know where to look.