Hi,
I have pihole running a long time. Yesterday I would install owncloud on the same pi. Everything went well without any errors, but I cannot use the webif of pihole.
DietPi v6.22.3 : 06:00 - Tue 30/04/19
─────────────────────────────────────────────────────
/admin gives me forbidden.
How can I fix that?
I just reinstalled pi-hole. I can access the admin page now, but /owncloud shows me only the placeholder from pi-hole, so no access to it 
access.log from lighthttpd
1556602274|192.168.179.230|GET /admin/api.php?summary HTTP/1.1|200|486
1556602275|192.168.179.230|GET /owncloud/ HTTP/1.1|200|405
You installed Pi-hole via DietPi-Software? It looks like your webroot gets adjusted (switch between /var/www and /var/www/html) but as long as the Pi-hole installed has not changed its method that should not be the case when installing with DietPi-Software.
all installed via dietpi-software.
it seems, that Pi-Hole + everything does not run ootb in diet-pi, right?
No, it should run out of the box. But I think, there are so many possible compositions of software …
For me, I get the pihole message in the web root, admin at /admin/ and access to the other pages in the /folders/
Good luck