I have a very similar problem. I am also getting a message that says "Your virus definitions are out of date. You should update them now". But on the overview tab, it says that everything is up to date.
And when I try to update the definitions manually it says "Unknown error".
This problem started to occur after I installed "anti-WannaCry" updates for Windows (KB4012212, KB4012215). This might be just a coincidence, but I reckon I should mention this.
I tried to repair Avast, as Asyn suggested, but with no luck.