...without holding anything in your hand.

So when you take your gun out and press the RMB to aim down the sights and then you put it away, you can`t jog anymore, only walk slowly or run. If you want to jog again, you have to take your gun back into your hands and then your character should automatically aim down the sights if you have put your gun away while you were aiming down the sights. Then you just have to press RMB again and put your gun back into your inventory and you will be albe to jog again.

The other way to initialize this same bug is by pressing the RMB without holding anything in your hand(s). Your character will 'invisibly' switch to aiming stance which causes your character to walk slowly. When doing it with a weapon, your character normally aims down the sights and starts to walk slowly, like it is supposed to be. You must right click again to jog normally.