I have had exactly that on mine - twice.
Once under warranty so replaced by MMC under warranty with a new box, then post-warranty on the replaced box.
I paid the factory to fit a new seal - all ok so far but not put too many miles on it since....
I suspect a link to filling up to the breather hole, not the overflow but can't prove it.
Last edited by nippymog; 27/01/20 12:03 PM.