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/timthumbLtGTc3

Query String : w=600&h=600&zc=1&a=c&src=https://www.campinginternazionalefirenze.com/img/placeholder/news.jpg
TimThumb version : 2.8.13