A TimThumb error has occured

The following error(s) occured:

The remote file is not a valid image. Mimetype = 'text/html' File = /var/www/html/timthumb/cache/timthumb1SDPJd

Query String : a=c&h=600&src=https%3A%2F%2Fwww.campingpanoramicofiesole.com%2Fimg%2Fplaceholder_offerte.jpg&w=600&zc=1
TimThumb version : 2.8.13