I figured out the problem. I had to delete the planet-150906.osm.pbf, after that I restarted the program and it started right up and downloaded everything.
I did run into another problem where my older Garmin can only use at most 2GB memory card, so I had to reduce the map area to fit to the memory card (resulting in a 1.4GB file). A few hours later it created the new gmapsupp.img, copied that to the memory card directly, and my GPS sees the maps with no problems.
A bit of a pain but it works now, although the only area I have is lower half of the eastern US, which should be fine since I usually don't travel beyond that.