You misunderstood my point: Pi-hole has to work on port 80 to be able to block any ads.
According to the Nextcloud website, Nextcloud is running on port 80 as well - it is not possible to run two services on the same port on the same machine without problems.
If you still want to run both on the same machine, go to /etc/apache2/sites-available/000-default.conf and change
<VirtualHost *:80>
into e.g.
<VirtualHost *:8080>
You will then be able to access Nextcloud as before on pi.hole:8080. (A restart of the system might be necessary).