Hi there!
It seems I just had a similar problem. I wasn't able to save my content due to a terminated session of the front-end Editor. The message said that I had to log in again but ... I found no way to restart the/a session. But ...
I was able to save my content by exporting the site (page Options -> Export). I saved the .json file on my drive. This was possible without being logged in.
Then I opened a new session of the frontend-editor in a new window: The Site shows my old (!) content. Then I imported the .json File I just created by the export-function. (page Options -> Import->choose File).
After this I was able to save the just imported content within the new session. Now the site is showing the new content.
Very happy, that this happened just whilst I made some testings! :-) ;-)
Hope this will work for other people, too!
Greetings Christian