A TimThumb error has occured

The following error(s) occured:

The remote file is not a valid image. Mimetype = ''/var/www/vhosts/akor.nl/httpdocs/includes/cache/timthumbtTVUjg

Query String : zc=3&w=1024&h=768&src=https://www.bandwerkplus.nl/klanten/www.akor.nl/uploads/project/thumb/406.jpg
TimThumb version : 2.8.14