Anyone on my server that tries to move water from a water bottle/jug into another container type gets kicked for this reason.

Some people have reported this when using the updated stack size controller. If you have recently updated then update again make sure you have the very latest version or try a roll back.

I used the latest version but the plugin was causing issues so I went back to the version before he made the big change and it's been doing it. I suspected it may be that plugin so I even unloaded the plugin and it was still doing it.

did you restart the server or just unload ? may be worth a restart just in case

I restarted also and still does it with old version