I have the same issue. It occured mid game, and have been unable to sprint since. I have reinstalled the game twice now, reset every single option available in game and am no closer to being able to run with my gun in my hand. A knife and equipment let me sprint, but currently after 70h on the game I'm putting it away in the corner until this is resolved. This is a fundamental movement mechanic and I am extremely disadvantaged to the point where it is not an enjoyable experience. I hope this is resolved ASAP because if it isnt I probably wont return.
