Cannot add images to static pages through the editor

[johanbove]

I am trying to add and upload an image to a static page using the editor. It does not work with an error shown in the browser console. This seems to be TimyMCE related?

ERROR: Error: Errors: 
Failed path: (dialog)
Could not find valid *strict* value for "body" in {
  "title": "File Manager",
  "url": "https://social.johanbove.info/filepicker/?type=image",
  "width": 650,
  "height": 550
}
Failed path: (dialog)
Could not find valid *strict* value for "buttons" in {
  "title": "File Manager",
  "url": "https://social.johanbove.info/filepicker/?type=image",
  "width": 650,
  "height": 550
}
Input object: {
  "title": "File Manager",
  "url": "https://social.johanbove.info/filepicker/?type=image",
  "width": 650,
  "height": 550
}
...

Using Known v1.2.4 on Firefox.