Comment 52 for bug 633352

Can you set the preferences "calendar.debug.log" and "calendar.debug.log.verbose" to true, and restart Thunderbird. When the reminder pops up, before attempting to dismiss it, click back to the main Thunderbird window, and go to Tools -> Error Console and Clear the existing contents. Go back to the reminder dialog and try to dismiss the reminder once, then post the messages that appear in the Error Console.