I think we'll need to see your ability editor settings to diagnose what might be wrong, but check the AOE (which controls the barrage range) and max. damage, and double-check that the requirements are fine.
1. If the tower's range is more than 500, it may be attacking while there are no units within range of Barrage.
2. If the tower has any base damage, that will be included in the dealt damage...so its 18 max. damage may be being reached before the appropriate 3 targets are hit.
Hmm, try recreating a steam engine on the humans, simply copy its stuff to your hero, and see what happens, the attack stats of course. Give it your barrage, and if it works, rule out things until you got what made it error.
I think leave it for real old news that cannot be sorted the reason we had it in the first place was a server issue that we do not have anymore. I don't want to get rid of the archive just not put anything in it for now until we figure it out more