I had a problem with a user account (file was locked) and the user could not delete it. It would be helpful to see the status of current issue on the user view. It is possible to use a support view for a admin account (or something else) to analyze the user actions and see it live?
How did you manage user problems and tickets with nextcloud as a supporter?
With redis a file locking cache, such errors should not appear: File is locked - how to unlock
In general, even if there is a locking, it should be removed after some time. If it isn’t it could indicate that your cronjobs are not properly configured (the admin page should show you when the cron jobs were run for the last time, it should be less than 15min) or a client is blocking files all the time.