Adding this in case it would help anyone:
If you are dealing with a markdown document that cannot save, there is a possibility that it’s caused by an invisible half-deleted unicode character. Better not attempt to find out where the invisible culprit is, since it is, well, invisible.
The practical solution is to screenshot the editor pane (use multiple screenshots if needed), extract text from the screenshot(s), and then paste the extracted text into a new document (or optionally, overwrite the current document).
