API error 400 when submitting assignment

[bfelix]

Hi all, our users have recently started receiving this error 400 response from the API when our system submits an assignment. It doesn't happen necessarily to all users, but more and more so recently.

The weird thing is the assignment is still successfully submitted. So it works...but we get an error response. Needless to say its a bit confusing and difficult to code against. 

I dont think this is quota related as I've read elsewhere. Anyone have a thought as to what wall we might have hit, or other ideas?

Thanks in advance.