Hi Jeremy,
I am sorry to say I have never seen this response in an error message. However, we may be able to utilize some tools to get more information.
The first thing I would try (if possible) is to capture network traffic when this request is made. The second thing I would take a look at are the Server logs just like Jonathan suggested below.
Lastly, when I am faced with a situation where the error message is not giving me the information I need, I try to think of a different way I can replicate the call. Can we try reproducing this same call directly at REST? Or even switch the client out for a different one?
I know it is tedious, but what we really need is more information about what is causing the mysterious "JSON' error.
I hope this is helpful.