How completely reset Nextcloud with all settings and keys on hole server - everything can be deleted

I am currently learning Nextcloud and am happy about this forum;
has already helped me with some important information

get the following error message:
“Wrong private key for the encryption app. Please update your private key in your personal settings to get access to the encrypted files again.” None of my passwords will work.

I have a dedicated server and installed Nextcloud in a domain there. Enabled default encryption and enabled server-side encryption. Before I could trigger backup, there were technical problems and the installation was defective.
I had to completely reinstall the domain and nextcloud installation including the SQL installation.
Can log me back in as admin in this Nextcloud, but he doesn’t take password with the above message.

Even if I install a Nextcloud instance for another domain, with its own DB and DB user, this message appears

How can I completely reset Nextcloud with all settings (I have no keys to restore) on the server and start again from the beginning?

If I don’t need to back up any data in the domain, it was only test objects.

Just open your config file in /var/www/nextcloud/config/config.php and set installed to false.
The next time when you open the webpage it opens a dialogue where you can reset your server.
In newer versions you have to create a file in the nc directory called something like ALLOWRESET. Just google for it, it changes from time to time.