Hey Tooserious,
Unfortunately the picture you were trying to post did not show up, but that said I have seen bad batches of ink cause overspray on Mimaki printers before. One trick you could try is to swap the dampers and ink lines between the yellow head and another channel that is working fine, like the Magenta. Do a couple of hard cleanings to pull the "wrong color" inks into the heads and then try the same print again - Yes, the colors are going to come out all wacky but what you're looking for is to see what color the overspray is. Does the problem follow the ink or stay with that Head channel?