yep, i've seen that thread long ago and as you say that issue should be solved, but.. this bug is quite different. The notification 'invalid target' is shown even on higher level mobs or players. let me describe it a bit.. you as a shadow priest have lets say a heroic dungeon boss in your target, you apply your dots, cast mind blast everything goes fine to the point you want to cast mind flay. You have clearly in your target hostile mob(in this case boss) but mind flay refuses to shoot and you get the 'invalid target' message instead. and to this day no core patch fixed that issue. What is most strange.. i discussed it with other priests and some have this problem and some don't.. so one can just guess what is the real problem..