[CLIENT] Can't edit Item Sets

I edit my item sets every day, but today I can no longer edit them.

What I can do: Create and name Item Sets and Blocks. View Item stats. Remove items one at a time(either with a right-click as normal or by attempting to move them) from the sets/blocks.

What I can't do: Add items to Item Sets/Blocks. Move items within Blocks or to other Blocks.

So all I can do is make empty sets and blocks, which is pointless. What happens if I try to move items that are already in sets/blocks? They get removed(completely deleted from the set). I have tried closing the Client and opening it up again but the issue is persisting.

The system that I'm currently using is a MacBook Pro macOS Sierra.