Found the fix (just tested).
Enable developer options (About Device > Tap "Build Number" 10 times). Then go and disable Nuplayer (its towards the bottom). (As soon as I disabled the player every story loaded perfectly)
I am having the same issues viewing stories... Videos chop up into screen shots quite often, though they occasionally work... it is extremely buggy and annoying to view stories on the S6 active (I am sure they will put out a fix soon)