It sounds like it's trying to ingest the file as a json rather than as a csv. Perhaps somebody touched the file with Excel and Microsoft converted it?
First, verify that the file is, in fact, a plain csv down on the box, by attempting to edit it with a vanilla editor (notepad++ or something like that).
Second, step through the upload one item at a time and read every instruction or message. This sounds vaguely familiar, and I'm pretty sure it's just either the file has been altered or an option that went wonky.