pressing s on any controller will key absolutely all attributes in the channel box, including level of detail switching controls which are often placed on a controller on the rig. Try and find this control and delete keyframes on the attributes that arnt effecting motion.

Its a good idea if you are just keyframing animation to use shift + w / e/ r to keyframe translation, rotation and scale.

In regards to the playblast... your camera isnt keyed? visibility isnt keyed on things in your scene? i know its obvious but just to check.

Other than that its quite odd unless that one file is set to read only, try playblasting to custom location.