So.
I wrote to B&O support, and they sent my a guide with a special software to recover the missing data.
Connecting to the product through webbrower, and uploading the software did the trick 🙂
Now the B&O app finds the product.
Don’t know if this is a common error, as the headline of the document says “How to recover ASE-TK product with serial number 00000000 (BeoSound 1/2, BeoPlay M5)”