I had Thunderbird set to auto update on my Windows 10 desktop and it updated to v91 at which point all the data in my calendar disappeared. I was able to prevent the auto update on my Windows 10 laptop and was able to also install an overnight backup of my desktop appdata\roaming\thunderbird folder there and get access to the calendar. I've tried exporting the calendar from the laptop and importing on the desktop but still cannot see any data. Many of the posts on the issue refer to Google calendars but what I'm using is the integrated calendar which I believe may be what started out as the Lightning add-on. I have tried creating new calendar events and saving those but they don;t show either. I'm not sure if the calendar reference in the developer tools output in the attached is relevant. Does anyone know if there is a solution to the problem and, if so, how is it implemented?