This is clearly not solved for the many of us who continue to get this error multiple times a day and are frozen out of our site for hours a week... I find multiple threads about this same issue and all say "closed" when the commenters are clearly saying it is NOT solved and I can vouch it has continued for me for over two months now no matter what browser (Chrome, Firefox, Internet Explorer).
We have disabled plug-ins and done everything commented on here and the problem continues with the same continuous message: "Connection lost. Saving has been disabled until you’re reconnected. We’re backing up this post in your browser, just in case." Then it often "saves" but takes out all formatting and changes text to one continuous sentence, requiring even more time (beyond all time lost frozen out of WordPress each day with this error message) to redo/fix all the formatting in a post.
You can tell us to open as many threads as you like and claim the problem is fixed but the fact that each user is saying it is NOT resolved and that this continues to be a widespread problem is beyond obvious. Why don't you let us know when it is ACTUALLY fixed system-wide or provide step-by-step directions to fix it beyond the useless steps that users are struggling to figure out for themselves?