Denon's fix, aka the adapter box, was easier to procure, design and manufacture, hence it arrived earlier. It took them 7 months from the moment the issue was reported by German tech magazine. The adapter box contains one chip MN864807 with one input/output enabled and all others disabled.
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Yamaha's solution, board replacement on entire line-up was more complex, as they needed to change several chips, aka entire board. The reason for that was that Yamaha had originally chosen to release AVRs will multiple inputs, whereas Denon opted for one speedy port. Yamaha offered free boards on 2020 models and withdrew A4A, A6A and A8A from release in 2020 in order to factory install completely new boards with second gen chips 40 Gbps on all ports. Hence more delay in fixing the problem on 2020 models and release of other three models in 2021. Further firmware delay is due to pending certification process, as explained above.