Update: the PLC9001 was indeed the vacuum pump. While I'm having trouble getting it to run consistently (any insights would be great) when it runs the error goes away.
PLC9004 is still stumped. I've gone into the PLC debug menu and killed/rebooted the PLC multiple times. It doesn't make much of a difference, although it brings up some I/O configuration errors related to one of the keyboards when I do that.