Destro = Right
Sinistro = Left

How about a photo of the offending pedal and threads.

If the threads are fowled with aluminum, then it may be able to be easily cleaned out with a knife or file. Look carefully at where the threads start.
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Ok, I looked at a few more posts and found the pedals. They look fine to me. Perhaps clean the threads in case there is any grit in the bottom of the threads.

I'd try the "S" (left) pedal on a different crank arm/bike. The problem could be with the crank.

Or does a different pedal work with that crank?