If you get this at startup, it is a sign of corrupt, missing or incompatible files in your client. These can either be the items.pak placed in the wrong location. It can be solved differently depending on the cause and whether or not you are aware of what is causing it.
1. If you made recent changes to the client, you might have an idea why it is happening. If you used the items.pak, restore the original and delete everything in your data/europe/items/items.pak folder.
2. If it involves any other corrupt/missing files, repair in the game launcher and or download it again if isn't working all of this