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/timthumbX3TNI5

Query String : w=250&zc=1&a=c&src=http://www.elex.it/img/rilevazione_presenze/schema_blocchi.jpg
TimThumb version : 2.8.13