Hi there,
If you have NICKNAME ERROR, please write in Bug reports, precising the player id and the new nickname.
If you don't have such an error, PLEASE DON'T WRITE ANYWHERE. I can't handle so many requests of changing the nickname here and in private messages, Please respect the time of developers.
And please don't try to trick me saying that you have such an error (but in fact you don't), saying that your friends can't find you because of nickname or send me cut screenshots.