Thanks for your help.
I made a complete reset (purge) of xserver-xorg, nvidia
and by nvidia-uninstall.
I choosed the nvidia driver free.
At reboot, victory ! It works.
Many configurations were lost by buster,
long time to return them.
I think (maybe) that the non-free driver of nvidia,
can not work with buster or the buster kernel 4.19.
Regards,
André