I have been experiencing this problem since I made the switch back to NextGEN Gallery. Periodically, about once a week or more, the thumbnail galleries disappear from the posts. In their place an error message appears "Gallery not found. Please check your settings." The gallery is surely there. I can enter Manage Galleries and select and manage any one of them. Another manifestation of the problem is the inability to insert new galleries into posts. It will not display any galleries to choose from. "Select Display Type" drop down will be empty.
I can bring them all back by going to "Other Options" and choosing Reset Options/Reset all options to default settings. But, I have to go back to each option and enter my parameters once more. I have done it at least 4-5 times in the last 3 weeks or so. I am not sure what it triggering this behavior but it is getting to be quite a nuisance. The worse part is that I have no idea when the galleries stop working for the visitors unless I go back and check them periodically.
I wonder if there is any fix, any suggestions to remedy this problem. I am running the latest version of NGG.
Back to reset routine!