Seems like that is a seam corner of the bottom cube face.
did you convert the original equirectangular image to cube faces and then correct the nadir ? You perhaps inavertantly changed the bottom cube face dimension ?
Check the bottom cube face dimensions to make sure that it is exactly the same pixel size width/height as the other 5 cube face images.. then convert it back to a full equirectangular image.
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EasyPano - Panoweaver