I see what went wrong. All bones in the model judge their position relative to the Root Bone and the root bone is moving backwards during your cycle. I bet that happened when you were making the walk more "treadmill-like" after having animated it in a more realistic fashion. It's only noticed onthe feet because you've successfully counter animt
So the feet are being pasted into a position that is mirrored, but releative to the root bone.
It is possible to convert a walk animated normally into a treadmill walk for a cycle, but very easy to make a mistake and get odd results.
I'd suggest starting over and remember to not move the root bone in an action.
Incidentally the arm bones aren't doing weird things because the are all FK and only repsonding to rotation information which isn't affected by the translation of the root bone.
Before you do more however... go thru the bones and make sure that all te center bones (including the hips, including the head) are really at X=0 in your model and that all the left-right bones are really exactly opposite each other. For example x=9.39 and x=-9.27 aren't exact opposites.
That said, while testing, I found that copy paste mirror could produce some unmirror-like results on the hips, which i can't explain. It may be necessary to tinker with mirrored poses to get them truly mirrored.