Some people prevents others from opening their maps damaging map archive. This map is still playable but you cannot open it in World Editor. In your case, message "Trigger data missing or invalid" means that author of this map removed trigger data file (using any "protector" or "locker") from map archive. This data is used only by World Editor, that's why you cannot edit it but map still works in Warcraft III.
For more info go to FAQ.