Your item config looks good and it is according to the info that I can find (I don’t use the sonoff myself):
Did status updates work before? maybe you changed something recently in your system?
“Long shot” note: If you recently upgraded your Mosquitto MQTT Broker Server to version 1.4.12, you may have a problem with the clientID naming.
But on the other hand… sonoff-977686 does not include ‘#’, ‘+’, or ‘/’ and you can send on/off commands… so this must not be the root-cause…
See: Redirecting...