I can't use all of my inventory

[worldofneil]

Have you ran Malikah's Well (or another dungeon?) recently and rolled for an "invisible" item?
Apparently... twice?

Adding to this, from the Known Issues After Patch 5.4:

The issues below have been confirmed.
We are currently investigating the issues and working to address them.

An item without a name is obtained in the instanced dungeon “Malikah's Well”

So I'm afraid for now you just need to wait, it will be fixed at some point.