It's possible to use Android tasks application and store these tasks on Nextcloud server. You can manage tasks in Nextcloud web interface and in Android application. To configure it you need install three applications:

1. Tasks on Nextcloud
2. OpenTasks on Android
3. CalDav-Sync on Android

For installation and more details use links below. Install all of them.

Official tasks application

OpenTasks on Google Play

For the synchronization we need an application. You can use CalDAV-Sync from the Google Play Store.

• create a list in Tasks application (in the Nextcloud web interface)

• add account in CalDAV-Sync application in Android
  • add CalDAV account
  • use https://example.com/nextcloud/remote.php/dav address if you host nextcloud on https://example.com/nextcloud/ domain
  • use your Nextcloud credentials
  • setup synchronization details as you need

In the OpenTasks application now you can see list created in Nextcloud web interface.

You will see tasks from Nextcloud server here. Tasks will be synchronized via CalDAV-sync. Newly added list will be also synchronized.