Cannot insert images in body of a basic page
I found a solution. Suggested by Tech-Tonic:

https://www.drupal.org/forum/support/post-installation/2018-08-24/ckeditor-image-button-cannot-insert-images-in-body-of-a#comment-12854570


  Does your content type have other file fields on it?
  If so, try temporarily disabling all file/image fields for the content type in "Manage form display" and save. Then enable them again and save. Worked for me, in Drupal 8.6.3.


As mentioned above, I also had an image/file field in the content type. Under 'Manage form display', I disabled the image field and saved. I then went back to 'Manage form display', moved it back to where it originally was and saved again.

After this the error no longer appears. This worked on the production, staging and development copies of the same site.