I found the error today!
It is due to an error raised in Blender that apparently prints, you had a “TODO” at this place about it.
I rewrote line 65-71 in proxyengine.py and replaced it with:
body_verts_weights = [[] for v in body.data.vertices]
for grp in body.vertex_groups:
for idx, vertex in enumerate(body.data.vertices):
vertex_groups = [g for g in vertex.groups if g.group == grp.index]
if vertex_groups:
body_verts_weights[idx].append([grp.name, grp.weight(idx)])
and the proxy fitting is working nicely without any errors while using Blender 2.79a and MBLab 1.6.1a.
Do you have your version control system accessible somewhere? I could make a patch/pull request.