I am having the same problem with my site, but I am seeing different results based on browser usage.
IE 8 (not sure about other versions) seems to show this problem when clicking on the gallery and the individual images are shown. When you click on one image that window then only shows that image with no navigation options and you have to use the browser back option.
Shown here then select a section such as General Meetings. Then choose a gallery and click an image.
When using Firefox and Safari I do not see this issue as the images work as intended.
I am very new to Word Press and NextGen Gallery, so I have followed other’s advice by updating the header and footer with the code shown in other forums.
This was working for me last night before I have made numerous site changes.
Any ideas?