If you are using Nextcloud 11.0.2 or later (e.g. 12), and logged in as root:
sudo -u apache /opt/rh/rh-php56/root/usr/bin/php /usr/share/nextcloud/occ maintenance:mode --on
mysql nextcloud
DELETE FROM oc_file_locks WHERE 1;
\q
sudo -u apache /opt/rh/rh-php56/root/usr/bin/php /usr/share/nextcloud/occ maintenance:mode --off