Stuck with barber pole and won't advance to next image

I have a photo gallery on my site that has always used Thickbox. Thickbox works great on this gallery, created a few months ago.
Today I created a new gallery, and clicking on any image in that gallery tries to bring up the Thickbox window with a barberpole that spins but will not advance to the next image. You cannot get out of it and must refresh or close the page.
The site does not have pathauto installed and is not using clean URLs. In comparing the URLs of images in the two galleries, I don't see anything out of place.
Thickbox has been upgraded since I created the original gallery.
Does anyone know what could cause this or what else I should check?