Hi Sorry to jump in here, but you guys seem to know what your talking about.
I am also having a invlaid JSON when uploading files. This takes places when students upload a file, when they are handing in an assignment.
The thing is that it is not across the board, it only happens to some students, file size does not seem to be a factor. The files are typically around 5MG. (I have set the allow limit much highier in the preferences).
The error appears when they select the file from their harddrive, however in a lot of cases if the student waits a while, the upload takes place, even though they are still given the error, and it does not appear to be successful.
I would love some help with this, as telling students just to wait and ignore the error is very confusing for them, and makes us look unproffesional.