The armature seems to import perfectly well despite this, unless the addon is trying to combine the skeleton with objects already in the scene, like when importing a .skin file over a .mesh.
Corrected.
At the point of the error the armature is fully imported. After that (what was not being done) the addon creates some drivers to reproduce the behavior of the armature in Haydee.
Any time I import a .skel file, I get an error message as shown after the standard output here:
The armature seems to import perfectly well despite this, unless the addon is trying to combine the skeleton with objects already in the scene, like when importing a .skin file over a .mesh.