There should be an error right above that message. It could be the nickname you are trying, you should use Latin characters only.
I've tried using "Omen", "Pilgrim" and "Dryad" for nicknames, as well as several individual letters.
There is no actual error message popping up in the server console. Just the Client sending a request and then "error". The SquadManager chat also doesn't show any error messages beyond the one I mentioned previously.