Sometimes, people try to add custom items to client and forget to make backups.
Our security not allow to change any files in system folder.
So if you did it or not but you cant login to the server getting error (make sure you aren't open in-legal software) you need to try make repair, download all updates till beginning.
How to make repair:
1. Close game launcher (if it's running).
2. Open game client folder (where you installed it).
3. Delete version file from folder.
4. Start game launcher (then all game updates will install again).