Andy,
Not entirely sure, but it might be something to do with the fact that you have got Mode S codes in the Fleet Watch Setup box?
As far as I'm aware, the only things that you can put in here are:
Aircraft registration - with wildcard
Flight ID - with wildcard
So the 43C*, AE0* and AE1* might somehow be causing the problem?
My Fleet Watch list is shown below. I've never noticed anything in there that shouldn't be there. I've left the 'Manual' setting in there, even though I don't really know what it does!