Sorry for the delay in posting the rest of this article.
My friend gave me the “you can’t use long cables with I2C” hint, which solved this problem.
The cause of this problem was that the OLED was connected to the ReSpeaker with a 30cm Grove cable.
The 5cm Grove cable I bought to try it out did not cause this problem at all.
Incidentally, the 20cm Grove cable that came with the OLED did not work.
So far only the 5cm cable has succeeded in making a normal connection, but this is a bit too short for my application.
The problem with the ReSpeaker has been resolved, but if anyone has any good ideas for using a slightly longer Grove cable, I would appreciate it if you could POST them.