Can't look left or right

The freelook settings for "pilot" and "on foot" are the problem.

The mouse movement bug can be fixed (while in game) without deleting or editing any files. The following is what I found corrected the issue.

In the Options menu click: Mouse & Keyboard, Vehicles, Key Bindings, Pilot, Movement. Use the (press and hold) ESC key to clear the "mouse bindings" for: Freelook UP, Freelook Down, Freelook Left, Freelook Right and leave them blank.

Then in the Options menu again click: Mouse & Keyboard, On Foot, Key Bindings, Movement. Use the (press and hold) ESC key to clear the "keyboard arrow bindings" (not mouse) for: Freelook UP, Freelook Down, Freelook Left, Freelook Right and leave them blank.

IMPORTANT: If you (previously attempted) to clear your freelook "mouse" settings for "on foot" and failed using any another method, you will need to rebind it (even if it still shows up) in the box under mouse. Just ignore the fact that it still shows and rebind it back to the mouse again.