Thanks. I have tested the Teensy with the sketch from Paul Stoffregen.
Also thank you all for the suggestions and hints. I re-soldered all pins (in particular D1 and D2) and carefully cleaned the board. To no avail. The only explanation I have...
Today I tried to solder PSRAM chips (APS6404L) onto a Teensy 4.1 board. Both chips did not work, but seem to be defect. The following error messages were reported while testing the memory:
First Chip error Message:
15:17:05.580 -> EXTMEM Memory...