Just had this issue also. Upon checking the supervisord log file as mentioned above by @JonathanM my error was.
FATAL: Module iptable_mangle not found in directory
The fix for me was this.
[info] unRAID/Ubuntu users: Please attempt to load the module by executing the following on your host: '/sbin/modprobe iptable_mangle' within unraid terminal.
Thank you for directing me to supervisord log file I was only checking the Docker container logs.