A TimThumb error has occured

The following error(s) occured:

Curl returned: 403
The remote file is not a valid image. Mimetype = 'application/xml' File = /var/www/html/timthumb/cache/timthumbaimPMt

Query String : w=640&zc=1&src=http://data-platform.naturalbooking.it/accomodations/321_6098_2529.jpg
TimThumb version : 2.8.13