I have been using ThunderBird on Linux Mint for some time now. I am using Outlook.com for emails, calendar and contacts and I sync with another laptop and mobile phone. All fine until a few days ago when I noticed calendar events were not syncing with my phone. I found one calendar event in ThunderBird had an error when I manually synced with TbSync. The error report did not say anything specific. I deleted this calendar event and TbSync ran successfully uploading 2 events that it had not uploaded due to the calendar sync error. I then wanted to manually recreate this calendar event in ThunderBird. When I clicked on "New Event", the Calendar New Event pop-up window did not appear. When I minimised the ThunderBird window, the New Events was not behind it. The ThunderBird icon pinned to the panel had a "2" in the corner. When I hovered over the ThunderBird icon in the panel, it showed both the ThunderBird window and New Event window preview. When I selected the New Event preview window in the panel, it still did not become visible. All I could do with with New Event preview window in the panel was click in the 'X' to close it. I checked for Lightning Calendar Tab add-on updates. It was up to date. I check the settings. All OK. I disabled and enabled this add-on. Still not working. My Windows habits made me shutdown and restart my device. Still Not working. When I checked the other ThunderBird functions, New Event pop-up windows appeared for Emails and Contacts but not Tasks which had the same symptoms as the Calendar.