Hi Jerome, and thanks for help.
I can't unregister my site and I always have the same error. But you are right, we use a platform each year and in august, we have made a copy of the old platform. My site seems to be registered because when I look at the "Registered moodle sites" on this page https://moodle.org/sites/, I saw it.
We have changed of domain name several times and the addresses of our platforms have changed too. Perhaps, this is the reason of the problem. I join a file with the description of our sites and addresses.
Best wishes
Dominique