OK. Experiment done.

I copied one "OK file" in a blank sheet and saved it with a new name
Ctrl functions worked in the new file

I copied one "not-OK file" in a blank sheet and saved it with a new name
Ctrl functions WORKED in the new file


That's a relief. I will copy all those (maybe half a dozen) files into new files and get rid of this annoying problem.

So, it seems that it has to do with some formatting or setting issue in those files.

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EDITED TO ADD:

I tried to do it again, but problem came back. I went back to the previous "NEW" file which "worked" . It didn't work again. So it seems the problem is here to stay. It is not a real serious problem, but it bothers me because it is something not consistent. Sometimes I can use those functions, and sometimes not.