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:
For installation and more details use links below. Install all of them.
For the synchronization we need an application. You can use CalDAV-Sync from the Google Play Store.
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.