All other board updates are working in my Arduino IDE. Just successfully updated the MBED package and the STM32 board manager, i.e. no network problem.
I could finally resolve the problem by:
completely uninstalling the Teensy board manager...
Since quite some time the board manager update in the Arduino 2.3.2 IDE on Windows 10 (same problem on Windows 11) does not work.
I want to update from board manager 1.57.2 to 1.59, but I am getting this error message:
Downloading packages...