Hi there ??
Usually, this kind of problem is usually caused by either a conflict with another plugin. The best way to determine this is to:
1. Disable all plugins except for NexGEN Gallery.
2. Temporarily switch your theme to a WordPress default one like Twenty-Twenty.
3. Repeat the action that is causing the problem
If you’re not seeing the same problem after completing the conflict test, then you know the problem was with the plugins and/or theme you deactivated. To figure out which plugin is causing the problem, reactivate your other plugins one by one, testing after each until you find the one causing conflict.
Cheers!