"You may not use nickname faruk"

Hello, is this still causing problems for you?

Could you please doublecheck to see if you can make only a single, tiny change to any other field?

Also, could you please try registering a new user and change any settings in that profile as well? If you then get a similar error, ("may not use nickname ...") this may be related to your computer or perhaps odd browser plugins.

More information about your environment would be interesting if the above debugging doesn't help.