image style (imagecache) images are not being created Currently i am unable to create images that get resized automaticaly according to the different image styles.

This includes the default styles such as thumbnail as well as the custom created styles.

I have checked the "Public file system path" as well as the "Temporary directory" (under Configuration > File system).

Both of the directories exist. 
The file permissions are good on the directories (777).

The original file that's uploaded gets created.
It's just the scaled images that aren't getting created.

Any help with this problem would be very welcome.
If there is already a solution somewhere (which means i searched for the wrong terms), a link to that would be very appreciated.

edit: 

I can't find anything in the logfile either, except for messages like :

type : page not found

message: sites/default/files/styles/thumbnail/public/fotoslider/slider_images/testbeeld20130103005.jpg
The problem was fixed when I changed the theme to another one, and back, as per @MPD suggestion.