I just had this problem and for me the solution was to create the folders and chown them:
cd /mnt/user/CABackup/
mkdir Appdata_Backup
mkdir USB_Backup
chown nobody:users Appdata_Backup
chown nobody:users USB_Backup
Cheers
I at least found a workaround here.
Create the folder and file /config/custom-cont-init.d/script.sh or in our case it will probably be /mnt/user/appdata/rutorrent/custom-cont-init.d/rmdirs.sh
The script will remove the folders on every start of the container: