Using Classic Outlook on Windows for personal email, calendar, and contact management
@Tim Cummins said
15:20:15 Synchronising server changes in folder ’Ajouter_�_votre_agenda.ics’
Looks like you have subscribed to a published Internet Calendar and Outlook can't get a recent copy.
Do you see the ...votre_agenda.ics at File -> Account Settings -> Account Settings -> | Internet Calendars | ?
Also check using Outlook Web https://outlook.cloud.microsoft/ for that Calendar under My Calendars or Other Calendars.
If you can't find it in those places, come back and leave a comment on this Answer, we'll look for some less likely sources.