Thanks for all for your advice.
So I decided to try to open the Ledger with Electrum. I'm running the installation wizard with these instructions:
https://support.ledger.com/hc/en-us/articles/115005161925-Set-up-and-use-Electrum

However now the wizard asks me if I want to select wallet file encryption? What does this mean? It says that "If you enable this
setting, you will need your hardware device to open your wallet".

But what if I've updated the firmware or the device has been reset? I guess the encrypted wallet file won't open then?

edit: I tried without the encrypted file. And pressed next. The wallet is totally empty. No history, no transactions, nada!?!
The network light (on the bottom right row) shows green. Also the Ledger light is green. What went wrong?