not possible to debug - configuration of GDB fails
I found a different fix. The VID/PID pair in stlink-v2.cfg was wrong. They had this:
> hla_vid_pid 0x0483 0x3748
but it should be this:
> hla_vid_pid 0x0483 0x374B
letter "B", not the digit "8". I looked up the correct VID/PID using Windows Device Manager...