int main() { printf("Hello world!"); }

Just tried the iPaq again after charging the battery. This time managed to get it to boot up. Guess the battery door, or rather the door latch, does matter. I think the battery door latch must be closed properly in order for everything to work. Pressing the door close by hand (without latch) does not work.

I have to take back and eat my own words about the battery door not affecting iPaq operations, but nevertheless, the supposedly fixed unit is still broken.