This guide describe how to deal with locked files. Is based on official FAQ from Nextcloud forum.
Edit config/config.php and change this line: 'maintenance' ⇒ true to false
Log into Turris via ssh and do these steps in a terminal:
mysql USE nextcloud DELETE FROM oc_file_locks WHERE 1;
edit config/config.php and change this line: 'maintenance' ⇒ true