I haven't adjusted individual item stack sizes yet, just the categories 

StealthyOrca

Same error

Failed to call hook 'CanStackItem' on plugin 'StackSizeController v3.1.4' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.StackSizeController.CanStackItem (Item item, Item targetItem) [0x00232] in <51f16a9fe2f044ba992ad25840e9dd97>:0 
  at Oxide.Plugins.StackSizeController.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x0073e] in <51f16a9fe2f044ba992ad25840e9dd97>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <3606d2af539c45e4b5c61658e6a8b307>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <c2afd8354b8b4f3ca451cf5a1aa111c3>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <c2afd8354b8b4f3ca451cf5a1aa111c3>:0

I'm getting this error every time I reboot the server and/or when I install the plugin. None of the custom stack sizes seem to be working for me either.

One player just moved some stuff from the inventory in her /backpack and then this error appears.. Maybe it is one specific item?

Hello guys. Yeah, same issue here. It happens if admin would add (more than 1) item with durability. Lantern, miners hat, water bottles, etc.

If someone would stack the same items, no error at all. Anyone knows how to fix this, or the maybe the author is working on it?

Thanks a lot. :) Keep up the good job!

i suggest roll back to what worked with your server guys and let the dev have time to work on it :)

Fixed in latest release.