I was wondering why I couldn't unequip items from my companions, or drag & drop items into equipment slots. The only way I could equip items was to double-click from my inventory - which means I had to first double-click equip a two-handed item in order to equip a new off-hand item.

I only disabled one addon as a test (the last one I installed), and it worked: Inventory Insight. Which is unfortunate since that is a really helpful addon