I see what the issue is. You've essentially modified the vanilla tank shell to be HE instead of AP without changing any other values. The vanilla definition of this shell uses hitAnimation: 26 and since it's damage type is 1 it'll use the vanilla HIT.PCK sprites indexed there. When you change the damage type to 3 it no longer looks for the animation in that image, but rather in X1.PCK and there's no animation indexed at position 26 in vanilla nor do you add any yourself via extraSprites, you only have the single "skull cloud" explosion at index 0.
Simplest solution, change hitAnimation's value to 0.
Fancier solution, add a custom explosion via extraSprites at an available index and reference that index instead.
PS: you probably want to change the hitSound value to 0 too. No crashes here, but it'll (probably) be weird to hear the metallic "dong" sound when the shell explodes on hit.