It’s giving me this error
Server error
500
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I cant change mine either through the website. Different error though. I just get a white screen and it seems to partially save my new name when I go back, but nothing get saved.
Have you tried downloading the Nexon Launcher and changed it through that? I keep having login issues with it.
Hey guys! White screen = you need to delete cookies! Might also fix the 500 error. 
Woo! Thanks @NxA_Noxie all fixed. Might want to add this fix to the closed sticky thread about changing your name. I had sent missmurder a msg asking her to change it as a last resort but it was low priority so I wasnt going to pester her about it.