I was a little too impatient! Indeed, after deleting a file needed for synchronization (I don’t see any other explanation), synchronization hadn’t been done for a month. Yesterday, after the complete reinstallation (for another reason), during the first few hours, nothing. However, this morning the synchro was active again!
So the reinstallation solved the problem, I think that a deletion and re-registration of the account at the phone level could have solved the problem.
I would like to point out that before that, I had never had any synchronization problems.