Good news. I received a new belt from Martin (Dillen).
The dealer could not deliver the belt. Instead he offered a new tape drive for € 250 or a refurbished drive for € 140. That's almost the value of the complete system...

Anyway. I fitted the belt into the drive, cleaned the system, reassembled everything and the Beocenter is back to live!

A 20 yr old tape sounds amazingly good....

A big "thank you" to Martin!