Preview to crop disappeared

https://www.check-in.dk/wp-content/uploads/2016/05/post-thumbnail-editor.png

That error usually happens when there is a conflict with another plugin. I would try disabling all other plugins and seeing if the error goes away. If it does you can selectively enable your plugins to determine which one is causing the problem.

The error could be as simple as having an extra line at the end of a file. See this link: https://www.ads-software.com/support/topic/image-not-loaded-in-editor-image-cant-be-displayed?replies=5