Oh, I see… I think I have the same problem as in this thread:
The OTP memory jumps to 0x20000000, which is supposed to contain the boot loader which jumps to the user program at 0x20400000. Using gdb I can see that the demo_gpio program is indeed flashed happily at 0x20400000, but the start of flash at 0x20000000 where the boot loader should be just reads 0xffffffff. There is no boot loader.
I guess that explains why the program will run once after I flash it, because gdb leaves it running from the program start address… but after I reset it the board goes back to doing nothing.
So, is it possible that some boards shipped without the boot loader flashed at 0x20000000? I’m pretty sure I didn’t accidentally erase it…