I get it done by changing the SYNCTOKEN_PREFIX form the SabreDav Doc to the nextcloud one according this link :
http://sabredav.org/ns/sync/ SYNC-TOKEN => http://sabre.io/ns/sync/ SYNC-TOKEN
Hope it helps.
Now, all is well.
I used 2 free applications that I deleted because the automatic synchronization no longer worked.
With DAVx5, everything works perfectly.
It’s user friendly and very simple to use.