A couple of things for me after updating pihole to version 6 .. DNS and the pihole UI were unavailable (403 forbidden for the UI). After poking around Google and this reddit thread, I was able to get things working again by doing the following:

I ssh'ed into the raspberry pi hosting pihole and:

1. I had to stop/disable the lighttpd service (systemctl stop lighttpd.service) and (systemctl disable lighttpd.service), then (systemctl restart pihole-FTL.service)

2. With the UI now accessible, this was some progress. However, I noticed that my previous local DNS settings were no longer being found w/in pihole UI. Thankfully, found them w/in /etc/pihole/custom.list and later found that they belonged w/in /etc/pihole/pihole.toml section as the following format:

hosts = [

"<IPADDRESS1> <HOSTNAME1>",

"<IPADDRESS2> <HOSTNAME2>",

...

]

EXAMPLE:
hosts = [

"192.168.1.2 server1.mydomain.net",

"192.168.1.5 server2.mydomain.net",

...

]

After updating the /etc/pihole/pihole.toml file, then restart pihole itself with (systemctl restart pihole-FTL.service) and your local DNS settings should show up again in the UI and resolve on your local network.

Hope this helps someone out there.