Actually, I did fix it. I did turn the sync off but still received the error. This only occurred after repeated, "Popup contains invalid value" errors. So, I modified the map by making sure that the primary key of the shapefile (feature service) wasn't in the pop up any more.
This cleared both issues so far. I have not turned sync back on to see if it creates the issue again though, but will probably try at the end of next week.