Hi Fabio,
we've checked it and the problem seems to be caused by some Javascript that is injected in your pages (I googled it and I think it's New Relic).
One of your notifications is from a forum post (notification id 1351). All notifications return a field named "fullmessagehtml" with the full notification message including HTML tags. In this notification, the HTML code includes the Javascript I mentioned above. For some reason the double quotes in this Javascript aren't escaped, so it breaks the JSON format and the app isn't able to process the response (hence throwing the error).
You should check where does this Javascript come from, and also why it isn't escaped by json_encode when returned by the WS.
I'm going to send you the full response via e-mail. I hope this helps you fix the issue.
Kind regards,
Dani