This worked for my problem:


https://social.technet.microsoft.com/Forums/ie/en-US/9e541e79-2011-42c7-ac9d-db7915e3c41a/outlook-2016-or-365-calendar-reminders-not-popping-up?forum=Office2016ITPro


Here is the specific fix described on this forum:


Does it works if remove WindowsPos key in Registry?

Please try:
1. Close Outlook.
2. Press Windows key + R to open theRun command, type regedit and press Enter to open the Registry Editor.
3. LocateHKEY_CURRENT_USER\Software\Microsoft\Office\xx.0\Outlook\Options\Reminders(where xx is 16 for Outlook 2016, 15 for Outlook 2013).
4. Find and delete theWindowsPos name from the right pane.
5. Restart Outlook.