I found a plugin conflict with the All In One Event Calendar
My NextGen setup includes a main album > sub-albums > multiple galleries
The gallery page is set up to display thumbs of the sub-albums.
When going to Gallery > Album > Edit Album > Select a Preview image the dropdown refused to load/open and there was no way to set a Album Thumbnail. Therefore no Album Thumbnails displayed on the gallery page.
By deactivating the All-In-On Event Calendar plugin the issue was resolved and I was able to select a preview image for each sub-album. When I reactivated the Event Calendar the dropdown refused to open again however the thumbnails remained set.
Next Gen version 1.9.6
All-In-One Event Calendar by Timely 1.8.2