Could you send me:
Which versions of Custom Item Manager and Custom Item Definitions you have installed.
What you were doing just before the error, step by step, so I can try the same thing.
Which custom item caused it. A screenshot of its settings in /cim would help.
Which inventory or container you were moving the item from and into?
Does it happen every time, and with other custom items too?
Does giving yourself a new copy through /cim cause the same problem?
Your CustomItemManager.json file from carbon/data/ or oxide/data/. This contains your saved custom item settings.
A copy of the complete error from your server console, including the lines just above it. The above error is incomplete
Do you use any backpack, stacking, or inventory plugins? Please send their names.
A short video showing what happens would be helpful too. Please back up your files before changing or deleting anything.
Are you using Oxide or Carbon ?