I had the same exact problem and it drove me totally crazy! I did manage to solve it in the end though, but it's hard to say what the actual problem was.
After connecting the polybrute to Polybrute Connect I did the automatic update and it suddenly reverted back to firmware 0.2.0. I had no idea why. After that nothing worked anymore and each time I tried to update the firmware I got an error. I tried the same steps as described in this post over and over again and nothing worked. Finally I contacted Arturia support and they basically replied: "The unit is stuck in "Bootloader" mode. To fix it, update the firmware and it should work again"
Which obviously I had tried already a 100 times on various PC's, laptops, using 5 different USB cables, etc. So that wasn't really helpful at all lol
After spending days on trying to fix this without any result whatsoever, I finally decided to buy 2 more brandnew USB cables in various (short lengths). The first one I tried didn't work again. I was about to give up again and tried the last cable and to my huge surprise that suddenly seemed to work! It was a very short USB 2.0 cable, about 1 meter in length. Nothing special about it. I tried various other USB cables of the same length before that and those didn't work, so to be honest I think is more pure luck than the actual USB cable. Hard to tell! But who knows, maybe it was actually the cable.
Hope that helps in any way for those that have the same issue!
