There have been a rash of people that started having problems with the Ancestry Enhanced Image Viewer out of the blue about a month ago. One night from one search result to the next it just stopped working on me. Everytime I tried to view an image, Ancestry.com got caught in an endless loop trying to install the viewer, even though it was already installed. Deleting my Ancestry cookies quickly solved the problem for me.
I need to ask around at work and see if the exact cause(s) has been identified. But today I noticed the Help system has a new article titled, Troubleshooting the Enhanced Image Viewer. If deleting your cookies doesn't solve your recently-surfaced viewer problems, try the different methods described in this group of articles.
If for some reason that link doesn't work, here's a posting on the Ancestry message boards giving detailed instructions on how to manually de-install and reinstall the viewer.