Are you syncing to any devices or other programs? If you are what protocol are you using?
 

This has been fixed and will be in the next minor build. The issue was syncing of Reminders on iOS through CalDAV which before never worked because we never had the ability for a client to create a calendar/reminder list but now with the most recent update clients can create calendars. We don't support reminders through CalDAV at the moment so a check was added to prevent the creation of reminders on CalDAV.

 

A temporary fix for those waiting for the minor can disable syncing of reminders in your iOS caldav account settings.