I don't know anything about the said incident but the problem with de-icing fluid in Avro's elevator system is very well known. Other operators have special procedures in place after having very similar problems, i.e. 2-step-de-icing and regular inspection. Type IV contains more thickener than type II, so if you use it the same way like II you get some very nasty solid residues inside the rudder/elevator.
Dani