This got it working for me. Tried every combo of different security permissions but it didnt work.. Put these commands into unraid terminal after starting the docker container
#Create blank config.toml with correct permissions
touch /mnt/cache/appdata/cubebackup365/cfg/config.toml
#Change permissions on folders to owner root.
chown -R root:root /mnt/user/appdata/cubebackup365
chmod -R 777 /mnt/user/appdata/cubebackup365