Can countries A and C be the same one?

No that is A and A which is not a transit and not allowed on Spanish passport. You need to continue to a third country on the way back to Japan to be eligible. Like Japan > Ningbo > Korea > Japan for example

Thanks for your response! Bearing in mind the information you have just provided, I have one more doubt: if I fly from Ningbo to Korea and then to Tokyo, do I need to have accommodation in Korea to apply for this visa exemption? Or is it just a matter of having flights in which countries A and C are different?
Thanks again!

Connections of an hour are treated the same as if it was a whole week. You just need to travel through the third countries airport for it to count.
If you flew Spring Air Japan from Tokyo and then Air Busan through Seoul or Cathay Pacific though Hong Kong then it would qualify.