Hi @WillowMobileSystems and @praveen,
@WillowMobileSystems I think I was not touched with that issue, fortunately : none of me or my first user had the warning message at authentication.
I tried what you say : to log out, then log in (even with two different google accounts), but the issue remained.
@praveen the folder (including google sheets) belonged to me, and my first user could at first use the app ; in the opposite way, once the app belongs to him and with me as a co-author, I cannot explain we can not sync it anymore ? Must be what you describe, but I’m afraid I will make a mistake at trying to share it.
EDIT : I tried a different way (very hard-making way) : I logged in as the new owner, then copy the app from here, and shared it back to me as “dev”. I cannot explain why, but that worked.
It must be something related to what you describe, but that goes beyond my understanding.
However, my main concern was for it to work, so…I’ll deal with that.
Thank you so much to you both, with your two suggestions I finally successed in solving it.