I would suggest to cast the same model in some COM based language (such as a VBA macro). Perhaps this is something with your .NET Interops registration on your machine.
I have to remind that the Addon is developed with SolidWorks 2013 and when using it in SolidWorks 2014 with a part consisting of a mirror stock object the above error occurs. Due to the fact that the COM sub system knows the name and guid of the interface I don't believe that the Interop registration on that machine is damaged. (Especially because derived parts without mirroring using the same interface still working fine.)
I would rather guess the feature definition of a mirror stock object doesn't implement the IDerivedFeatureData interface anymore, perhaps it implements another one.