trouble getting Linux Mint 20 to detect my Digispark

[piriroter]

Hi,

I followed the instructions in
https://digistump.com/wiki/digispark/tutorials/connecting

esp. pasting this URL into prefs| add. board URLs:
https://raw.githubusercontent.com/digistump/arduino-boards-index/master/package_digistump_index.json

After downloading I expect that the Digispark stuff appears in the board manager (under contributed as stated in the tut.)

But that is not the case. But all other projekts e.g. Arduino mega... works
What now?

greetings
pi