I've had this problem for months and could find nothing specific online, but I believe I've now solved it. Right click on the Drafts folder and select the properties option, followed by the Repair Folder button. In my case nothing seemed to happen, and the properties window remained open. I had to remove it with the X close tool, top right of the window. At this point a couple of years old drafts in the folder list were now present as unreadable faint pink blocks. A bit scary, had I made things worse ? I closed and reopened TB, when I was presented with a Welcome to Thunderbird message, and the default message layout view; surely a cause for concern ! However my two old drafts were now restored, and a quick compose trial showed that auto saving my drafts was restored, without the annoying error message. Finally, I easily restored the layout to my liking. So it looked like the repair folder had worked and needed a restart to complete. Hope this helps others.