In the same club however when reading a little more detail noticed another possible cause.
Quest = Complete 'The lost Skull' without using any cavalry, but when you click on quest it expands to read -
Complete the Lost skull only using the following units: Recruit, Militia, Bowman, Cavalry, Elite Soldier, Crossbowman, Cannoneer.
So in the list it says we can use Cav & the only unit not listed is Soldier... Errrrrrrrrr so possibly not worded correctly?
Edit wow this thread grew quick, left open tab & posted late to see already stated previous