If you are running a pump and trying to fill a barrel and the barrel doesn't fill, or your client crashes while trying to give/take water from a bucket or a jerrycan on a water barrel... go into the config file for StackMultiplier and change the following:

"BlockedList": [
"hammer"
  ]
}

To:

"BlockedList": [
"hammer",
"water"
 ]
}

I was sooooo happy to figure this out!  Hope it helps!