@velkkis, try flashing your n950. After that Ancelad's repo mirror should help you with enabling dev mode. At least he says that he tested it that way on his n9.

Btw, before the flashing try to check for updates in Settings -> Applications -> Manage applications -> Updates -> *click the icon in the top-right corner* with installed repoMirror, and after that try enabling devMode again. If that fail, well, I think you can do nothing but flash your phone.