Issue: I can't switch loadouts due to an error: "Unable to equip some items from the loadout".
Cause: Loadouts are affected by all aspects of your character: mounts, companions, gear, enchantments, runestones, potions, etc. If you move any of these items to other characters or delete items from these loadouts, the loadout itself cannot change because it cannot detect the item that was previously saved to it.
To fix the issue, use the "Apply to all loadouts" option on your gear, companion, and mounts. Afterwards you should be able to switch loadouts again.