So I contacted B&O Support and the solution appeared to be very simple: go into the Maintenance menu and Delete Databases! After that BeoMaster 5 reboots and starts downloading the complete actual N.Radio database again!
And this worked fine for me: after an hour or so I got 67397 ‘new’ stations re-downloaded from the portal. Some hours later the logos were also present. My failing stations play again now and I see plenty of new stations appearing.
So thanks to B&O Support for their clear, simple and effective solution :)
This is the link to the document: