Yeah once I press crouch(+duck or ctrl) then speed(+speed or shift) i can't move forward but if i press speed then crouch i can do it (if the game mode allow speed button which Zombie survival doesn't by default).
On zombie zone server this combination of keys are essentials.
But if i bind the crouch to a mouse button (Mouse4 for example) i can move backward
#i cant sprint backwards
silk karma
covert citrus
Do you have "animated sprinting" installed?