Comment 8 for bug 1273652

I'm having the same problem too. This is recent, so I suspected an update may be the culprit. I have tried reverting to a previous version of nvidia driver, but that made no difference.
Error message is substantially the same-

ERROR: Error querying target relations

The program 'nvidia-settings' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadRequest (invalid request code or no such operation)'.
  (Details: serial 82 error_code 1 request_code 154 minor_code 29)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)