Lost USB connector. I wrote to technical support - they answered that it was allegedly unique and inimitable, so sorry, your keyboard is now a brick. But this is not true. You can buy any peripherals with the same connector, for example, a Logitech M185 mouse (I bought a used one for 400) and use the Logitech Options program to connect the USB connector to the K470 keyboard and mouse. Actually, I did just that, after six months the flight is normal. This solution has long been posted by users on the Logitech support forum, but the support staff keep repeating the mantra that nothing can be done.
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