title_body: Highlights and notes are not save to my Clipping file
upvoted_answer: Ok, much to my surprise, I solved this.
  I had to delete my (empty) Clippings file, then connect to the internet, then make a highlight. Apparently the issue was somehow caused by synchronization, because after doing this my highlights and notes appear on my Clippings file again, even if I'm not connected to the internet.