PossumOpossum Have you created the Nextcloud credentials in DavX5 which points to your Nextcloud instance?
DavX5 is an Android app designed to interface with Nextcloud, for three things mostly: Contacts, Calendar entries and Tasks. The commonly used one is Contacts, using Nextcloud's inbuilt CARDDAV Server.
Once Nextcloud is installed on your private web server/domain (for instance: yourdomain.com/nc/ or yourdomain.com/nextcloud/), you'll need to assign a username (typically this is 'admin') and a password.
Once in, you'll need to add a few apps to Nextcloud's freshly installed platform including: Contacts, Calendar, Tasks, just for starters.
When you connect DavX5 to your Nextcloud instance, it will look for either a CALDAV or CARDDAV Server. You'll need to create an account using the "Login with URL and User Name" option.
Base URL: <quote>https://yourdomain.com/nextcloud/remote.php/dav/addressbooks/users/admin/contacts/ </quote>
Password: <enter your password>
[this assumes your Nextcloud directory is named 'nextcloud' and your username is named 'admin'. If not, change the connection string to your own credentials whatever they are.]
Click Create Account on DavX5, you should now be connected to your Nextcloud Server.
Let us know how you get on.