After every reboot of Pi-hole I have to restore the database via pihole -g -r because elsewise I'm getting a resolvement error for localhost in the diagnosis output of pihole -d.
These database errors appear to be from your long term database, not the gravity database. Let's try making new database whole saving the old one if needed.
The files actually were found in /etc/pihole and I did what you suggested with the errors having disappeared now. https://tricorder.pi-hole.net/tnf0ulgy0r
But there's still an error:
*** [ DIAGNOSING ]: Dashboard and block page
[✗] Block page X-Header: X-Header does not match or could not be retrieved.
HTTP/1.1 200 OK
Content-Type: text/html; charset=utf-8
Accept-Ranges: bytes
ETag: "1251518635"
Last-Modified: Sat, 01 May 2021 17:40:05 GMT
Expires: Mon, 10 May 2021 16:18:19 GMT
Cache-Control: max-age=0
Content-Length: 10701
Date: Mon, 10 May 2021 16:18:19 GMT
Server: lighttpd/1.4.53