"Unknown Error" when trying to rig imported model

When I load my custom character into Mixamo and place the points, I get a message saying "Unknown Error while generating motion."  It's like Mixamo doesn't recognize my model as a riggable character.  I realize it may be hard to tell from just a screenshot, but on first glance can you tell what it is about my model that is causing issues?  The thumbs look reversed here, but they are correctly positioned. Also, I tried this model with different hands, thinking they were the problem--but no results.  Granted, it's clumsily built. But I just ran this off as a prototype for something in AR. Wondering if my construction method in C4D is the problem, but I haven't had too many problems so far.