Did you update PiHole to V6 after you have updated DietPi to latest 9.11?
Because we will move PiHole to a different port away from 80/443 to avoid any port conflicts with web server apps like Nextcloud and to allow both running next to each other. This procedure we did on the update to 9.11 for existing installation. But if you did PiHole v6 update afterwards, this step was not done. However we will do this again on next update. If you like to have Nextcloud running now, we can adjust your PiHole configuration to free up port 80/443.
As an alternative we could adjust the web server port to be able to LISTEN on something else if it is just for testing purposes.
For production operations I would recommend Nextcloud web server to take port 80/443