ballscrewbob:
I presume the USB HUB has its very own PSU ?
Unfortunately not, and that seems to be the crux of the problem. I just tried disconnecting the hub and connecting the Arduino solely to the 5m USB cable, and it was found at startup multiple times with no issue. So it looks like I simply drained too much power from the 5V line on that USB port, so the Arduino didn't boot properly.
I'll look into powering that hub now; hopefully it'll resolve the issue.