Michael,
I had this problem in my previous caravan, and initially in my present caravan.
The solution is quite simple. Stop useing the pink Thetford aquarinse, flush the remainder of the thetford from the loo, and use Elsan pink flush.
Myself and others have done this and we have had no further problems.I don't know why it works but it just does. I still use the Thetford aquakem (blue) in the bottom tank.
Hamer