relocate from playlist > "this file is already in the collection" > but ONE by ONE?

I have all tracks in my collection...but have none in my playlists...dragging ONE by ONE works...but why does recordbox not make a difference between collection and playlists? 

Where is the information stored (which db file)? 

I cannot find any way to influence it in settings.

Facts:

RB was moved from one computer to the next.

RB db (all information) was copied from first PC to the file folder of new PC.