I have the same problem .. Started today after installing 6.18.0.
But I still cannot reproduce the error. But it occurs in multiple emails now.
I wanted to reproduce the error to avoid it, so I started to create a new email. Saving works then. But after a time, it won't work anymore. But I don't know after what changes to the content of the email this error occurs or is triggered.
I checked the saved "body" in the database and there are no "html-errors" i.e. it validates.
But when I open the editor, to edit the email-content the editor tells me, that it found a "recovered version". That is in every mail, with this error.
What I noticed the following:
When I create a new "empty" mail from a template, and then I save it and reopen everything is ok. But when I leave the editor open for a certain time without changing any content (so it is nothing that is entered incorrectly), this error occurs. Saving is not possible anymore. If this error occurs once, you cannot get rid of it 🙁
Please support ..