Nextcloud Logfile Not in /var/log/nextcloud.log

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