Looks like the Camera based selection is on automatic mode.Try to turn it off in modeling toolkit. If as you said the option is greyed out, you can try to type this command and see if it does something or if you got an error message in the script editor:selectPref -useDepth false;
No problem ;)The backface culling selection is broken since maya 2016.5.I've reported the bug in the maya beta forum but nothing has changed.For the moment we're stuck with camera based selection. If you want, try to bind this mel script to a hotkey, it will toggle the camera based selection and display a message in the…