I am assuming the error you got from running boinccmd command (for future always say which command or gui action gave the error).
There is a known bug with boinccmd, it requires file gui_rpc_auth.cfg is the current directory, you can copy that file from /var/lib/boinc-client/ directory. The gui manager does this automatically, but it must have permissions.

That is not a bug, it is a security feature to prevent other users from using the manager to manipulate the client