It seems a some files may have been incorrectly updated because sometimes just deleting or using a backup file (in the cases with calendar issues ) fixes the situation. But there are cases when it is necessary to also start Thunderbird using 'Run as Adminitrator' which suggests a change in the permissions occured. When Thunderbird updated, maybe the permissions were not inherited, so the computer would not allow some files or folders in Thunderbird to run in the computer User Account.
Bug 1789326 Comment 23 Edit History
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It seems a some files may have been incorrectly updated because sometimes just deleting or using a backup file (in the cases with calendar issues ) fixes the situation. But there are many cases when it is necessary to perform file deletions AND also start Thunderbird using 'Run as Administrator' which suggests a change in the permissions occured. When Thunderbird updated, maybe the permissions were not inherited, so the computer would not allow some files or folders in Thunderbird to run in the computer User Account. Users have computer User Account set as 'Administrator'.