NextGen Gallery asks for login

After having quite some issues upgrading to WP 8.2.1 (blank screens etc.) I decided to start all over and did a fresh install in another subfolder on my domain.
I copied the Theme I was using from the “old” folder to the new folder. Uploaded all plugins fresh. Everything seemed to work fine but I have an issue with the NextGen gallery.
Every time I want to view a page in my browser that contents a NG Gallery a window popups asking for username and password. I can go to ALL other pages without any issues.

Any hints why this is happening suddenly?
Thanks
Michael