Where should I specify the UploadUrl?

I already changed the config in default.config, admin.config and guest.config. But it keeps showing the above error message "uploader require form if scope. UploadUrl is not specified".

After clicking the OK-Button, I can see the images in the specified directory (this also happens on "Image Gallery"). Upon clicking the "Upload Image"-Button nothing happens.

I would appreciate any help / hint regardint this.

Thanks and regards,