We have a client when trying to comment on a comment thread and hits "Save" he is constantly getting this error.
Unexpected end of JSON input
He seems to have the same permissions as others who do not get this.
Maybe your client is facing the same issue as Iain in this discussion
In fact, his proxy server was the root cause of his issue, and he tried to remove it and everything worked fine.
So, could you please ask to your client to do the same thing, or to connect to a network where requests are not filtered by a proxy server?