I had the same problem. Maybe it come from an old version of Timthumb :
https://www.websitedefender.com/web-security/timthumb-vulnerability-wordpress-plugins-themes/
I use The Dawn for my website. I change the timthumb.php to put the new version.
I’ve also check the wp-config.php file and removed the code.
It’s working now (without uploading a fresh version of WordPress). Wahtever, I will upload a fresh version of WordPress, just to be sure.
