Try this: * Menu app icon > Help > Troubleshooting Information * Under 'Application Basics' - about 11th in list is Profile Folder - Click on 'Open folder' A new window opens showing the contents of your 'profile name' folder. * Exit Thunderbird if running....this is important * Click on 'calendar-data' folder to see contents. * Copy local.sqlite file and put copy onto desktop. * In calendar-data folder - Rename 'local.sqlite' to 'local.sqlite.bak' - Download install addon - Firefox sqlite DB manager add-on into Firefox browser. * https://addons.mozilla.org/en-GB/firefox/addon/sqlite-manager-webext/ * Start the addon - you should see a new icon on the toolbar near the menu app icon - hover over icon and it will say SQLite Manager - clickon it. Use addon to open the desktop copy of the local.sqlite DB * drag the desktop copy of 'local.sqlite' into the opened addon. run two SQL queries - you type this in the yellow area Searched for items with null id: '''SELECT * from cal_events WHERE id IS NULL LIMIT 100''' If you find any... So then use the following to deleted it: '''DELETE FROM cal_events WHERE id IS NULL''' * Saved the database file, 'Save' located at the bottom - save is usually saved in the 'Downloads' folder. * Access the 'Downloads' folder to locate downloaded saved version of 'local.sqlite'. * put the saved 'local.sqlite' back into the 'calendar-data' folder restart Thunderbird and report on results.