Update: I think I fixed it 
With my original (old) ROCK database (smb://rock/Data/RoonServer) I quit Roon.app on my iMac, renamed ~/Library/Roon on my iMac to ~/Library/Roon_old and restarted Roon.app. A new version of ~/Library/Roon got created. Now the problem is gone. 
I just had to configure some of my settings (Settings/General) and the order of my bookmarks - everything else seems to be fine.
I will keep my backups for some time and will observe if everything runs smoothly. If yes, I will set this support issue as solved.