Hi Roy,
From your description, I am afraid it's your board hardware stable problem.
The Kboot we have test it on our official board, it's no problem. I think you can check your problem board's hardware, whether the GND is stable enough, you also need to check your USB-TTL device, whether the wire is too long or any other EMI in your test situation.
If your TTL-USB works for most of the board, just one specific board have the problem, I think it's your customer's board hardware problem. Please check your hardware.
Wish it helps you!
Have a great day,
Kerry
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