Tasks no longer syncing after upgrading to macOS 10.15 Reminders or iOS 13 Reminders
After upgrading to macOS 10.15 Catalina or iOS 13, you will be prompted if you'd like to "Upgrade your Reminders" to use newer features in the Reminders app. Clicking "Yes" seems to break CalDAV sync and these lists / tasks will no longer sync with third party apps over CalDAV. There is no known workaround for now, other than ignoring the request and selecting "No". Upon complaint, Apple has responded that this works as intended.
The only workaround would be to create a new List in 2Do (or a third party CalDAV client of your choice) and move existing tasks you've "upgraded" to this new list.