I followed the procedure in Ancestry's response and I can now use the Enhanced Image Viewer. The procedure even resolved the problem I had with the viewer when the Norton Confidential Internet Explorer add-on was enabled.
In "STEP II:" I wasn't sure which Settings button was being refered to for steps F to J when verifying security settings to allow proper coding:
F. Click on the General tab and choose the "Settings" button
G. In the next window, select the "View FILES" button
H. From the File menu at the top of the next window, choose Select All
I. Press the Delete key on your keyboard and click Yes or OK when prompted with a warning about deleting the files
J. Click on the OK button to close Internet Options
I chose the Browsing History Settings, but no files were displayed.
So, so far, so good.