For those who might come across such an error, fix is simple:
> Navigate to your Shaiya Program Folder and open it. [C:/AeriaGames/Shaiya]
> Open "Version.ini" using Notepad.
> Change "CurrentVersion = ###(this number varies according to updates), and change the number to a lower level [i.e., =221 to =151] and save the change.
> Run Shaiya Updater, let the game client re-update, and boom, problem solved.