I still think it’s cartridge related. Second guess would be a bad tonearm wire. One other check you can do is try to flip the channels by swapping leads and connecting the cartridge left ch to right and right ch to left. See what happens. If the problem moves to another channel, you most likely have a failed cartridge. If you have another cartridge you can try it would help.
This was a very good suggestion, thanks!
I agree with @knock1 - it’s the cartridge - because you have observed a signal coming out of both left and right channels - it cannot possibly be the wires
Thanks for this note that made me check my logic.
Yes, since I didn't change the channels in the phono input and both channels sounded in the two opposite settings, the wires of both channels should be fine (the same right channel that did not sound, it worked when the signal came out from the cartridge to that channel, and this happened because the channels were swapped on the cartridge - the left channel signal now went through the wires of the right channel).
Perhaps, since likely it is the cartridge, it is not the worst outcome.
But how the cartridge got broken after just 20-30 hours of use?
I have to look for a new cartridge, would you have any suggestions for P2?