I had the same exact problem! I had a healthy, beautiful plant and it just died over night! I dug it up, and found the same thing! I was thinking they were termites, but I was told it was unlikely. I'm still not sure I believe that advice, though. The bugs on mine looked very similar to yours, but I think yours are larger.
Out of curiosity, who did you buy your rhizomes from? You can PM me if you don't want to share that info publicly...