I can't access the admin page of Pi-hole using the dns name

Sorry, yeah I misread the debug log.

Looks like your IPV6 address has changed since you first installed pi-hole.. the one in /etc/pihole/setupVars.conf is different to the actual IPV6.

If Pi-hole is handing out the one in setupVars.conf to your clients, then they're going to have a bad time resolving domains...

Please see this thread for some information about that: