Error when inserting image via WYSIWYG

I have a site where I like to insert an image from the library to a page via the WYSIWYG. It works fine, but as soon as I preview or saves the page the image is not displayed but replaced with this:

{"type":"media","view_mode":"media_preview","fid":"112","attributes":{"class":"media-image","typeof":"foaf:Image"}}

It seems something is not configured right.

Can someone kind person help me please. I have used 4 hours researching but with no result.

Thanks,
kampmann