My drupal website uses thumbnails. I can see that the first thumbnails were generated fine, and the image url has an 'itok' token. Now, the image is not showing. However, if I copy and paste the url and remove the itok, I can see that it's there.

I've checked the logs and I cannot find anything. Could it have something to do with my linux applications? Or maybe with the upgrade from 8.1.7 to 8.1.8? Or maybe a problem with permissions?

Any ideas? Thanks in advance.