Key group stops working

Hello everyone,

I have been a happy M01 user for a few years now, but since a few months, four adjacent keys (tilda, Q, W and E) become unresponsive (LED stop working too) from time to time, and magically start working again after a few days. I have been searching the forum for similar threads, but all reported problems seem to be mechanical failures.

I am not an expert in electronics, but my guess is a faulty PCB, right? Does anybody know where I could start my investigations?

Thanks for your help!