I’ve got an image in the error’s content folder when I (manually) remove this image, the panel view of the error page shows the default blueprint page (Simple Page from the starterkit). When I move the image back into the error’s content folder the expected error blueprint is used again, with a markdown field. (The image was already deleted from the textfield prior to removing it from the content folder).
The image was placed before I added a files section. When I leave the image in the content folder and upload another image via the added files section first and then remove the image, the blueprint remains as expected and I can remove the image from the content folder.