If it were me with this issue, I would delete the backup folder:
rm -rf /home/user-data/backup/
Then I would rerun the mailinabox command:
sudo mailinabox
To get your first full backup, assuming you have backups configured to store on the box, you can then run:
sudo mailinabox/management/backup.py
This command will rebuild the .gz files you reference and hopefully having deleted the entire backup folder, your locking issue will be resolved. Keep in mind you will also have a new secret_key.txt in /home/user-data/backup which you need to backup to a safe location in order to use the encrypted backups you’ve just regenerated.