Hi Felix,
I had also this error almost every time when I first reset the AQ in boot mode (only red LED is ON) then click flash firmware and then clich OK. From my experience with QGC, I seen that I can always flash from first time if following this steps:
1. Select firmware
2. Click "Flash firmware" button
3. ONLY NOW reset AQ in BOOT mode
4. Click "OK" in the confirmation window.
Regards,
Sergiu