Strange ankle distorsion on shoes

Hi. When you duplicate and mirror an object, its weight information and uv placement in the uv_1 map remain the same.
Weights dictate how much influence every bone of the skeletone has on a specific area, or in other words, how the mesh will animate.
Uv_1 map dictates how the mesh will morph during body customization in CAS.
Vertex paint dictates what deformation map(s) will be used during body customization in CAS.


So instead of using Mirror modifier to duplicate the mesh:
- switch to Object mode (TAB), in Vertex groups tab delete all meshgroups for the left foot (they have L in their name) by selecting each and clicking on - icon. There will be a random group that does not belong in the feet area. Do not touch it. It does not affect any vertex, but the meshgroups in the blend you import in the package must have the same vertex groups as the original;
- press Shift D, without moving the cursor LMB-click to comfirm,
- select the duplicated version, replace all R in its vertex groups names with L;
- to edit the uv_1 map unfold rig, make feet visible and selectable (eye and cursor icons), select them, select their uv_1 map, select everything with A in UV editor, set a background image (that field after UVs), Shift-select your mesh, select its uv_1 map, select everything, set THE SAME BACKGROUND IMAGE, then View - Draw other objects. Now you will see the wireframe of the feet. Flip the uvs with UVs-Mirror-X, press G X and move them to match the left foot uv placement.

Anyway, unfortunately, cc making is not as straight-forward as one would wish. You need to watch tutorials first, not necessary on shoe-making, to understand the principles behind it.