Thank you for your answer.
I confirm that the log file is in data directory (as it always was).
I double-checked permissions.
As far as I can tell, the only thing that changed is the rotation made by logrotate.
My logrotate config file for nextcloud is :
/[datapath]/nextcloud.log /datapath]/audit.log {
su www-data www-data
weekly
rotate 12
size 52428800
missingok
notifempty
create 644 www-data www-data
}
I launched logrotate manually to check the syntax, it worked fine.
EDIT : I even changed mysql config value as suggested here