Thanks for your report.
This has been fixed with version 1.0.1.
On existing Davdroid setup where cardDAV and calDAV are correctly configured and running (on Android 5.1), is it possible to add the task functionality without to recreate the existing setup?
The answer is: NO!
Seem necessary to delete previous account, then reinstall DAVdroid and install a task manager app as OpenTasks, then reinstall DAVdroid and recreate the account.
From DAVdroid Configuration webpage:
If you want to use task synchronization, install the OpenTasks app [F-Droid, Play Store] before installing DAVdroid.
Download and install DAVdroid
Click on the “+” floating action button to add a new DAVdroid account
Enter your server details (see below) and click “Next”
Enter the name for the account. Basically, you can choose any account name like “Work” or “Private” or “My Account”, but when you use attendee information in events, Android expects the account name to be your email address. The account name will be used as ORGANIZER field for group-scheduled events.
Then, click on “Add account”
Navigate to the account in DAVdroid. The available collections (= address books, calendars, task lists) are shown there
Check the collections you want to synchronize
Optional: open account settings (from the action bar) and adapt them to your needs
As soon as a calendar or task list is synchronized, it’s available in your local calendar app(s) and OpenTasks
It’s possible since Android 6. Previous Android versions don’t support that.