You probably have the animation going down the spine too much. I can't remember which bone it is, but the body is basically "split" in half so that top and bottom animations can be run on the model at the same time. In this case, your animation probably just don't look good (due to the animation using too much of the spine) other animations are being played on the ledges.
As for the OJP animation file, if you want to add a new animation, you're going to have to merge your animation with the modifed ojp humanoid.gla instead of the basejka one. That way you won't be removing the additional animations that OJP already uses.
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