Solved. It was a problem with the SD-card. Everything works fine with a new one, formatted in the player.
If somebody is interested in details: My first HifiWalker (Firmware 1.5) had a hosted port, residing on the Kioxia Card which came with the player. You now warn against using these
. Everything worked fine for for 1 1/2 years. Yesterday I installed the native port on the device, using a small SD-card for test purposes. Everything worked. I then renamed the old rockbox folder on the original (Kioxia) card and copied the new .rockbox folder from the small SD-card to the original card. Rockbox was not found. I deleted the new rockbox folder and renamed the old one back to .rockbox: Rockbox started.
Today I bought a new SD-card, formatted it in the player and copied the .rockbox folder on it. Rockbox started. And now my only problem is that it takes ages to copy about 60GB of audio books to my new card.