broken theme preview / installation on wpmu to wp3.0 upgrade

I have four wp multisite installs, but this problem is only occurring on one of them, and I have tried in vain to figure out the problem. This particular install was an upgrade from wpmu. The other three are new installs.

New themes will not install from the theme installer, and I can’t get the preview window to open. instead, it opens in an iframe-type box at the bottom of the page.

Can anyone help me with this?

Thank you.