If I disable a selection of DNS, then I cannot browse, (I get Hmmm… can't reach this page
www.google.com’s server IP address could not be found.) which was not an issue in the past because I use Unbound as my DNS on 127.0.0.1#53
Now, in order to browse websites, I must select a DNS in the DNS list.
...
[✓] udp:0.0.0.0:53 is in use by pihole-FTL
...
[✓] tcp:0.0.0.0:53 is in use by pihole-FTL
127.0.0.1 is the loopback address (an internal IP address that sends data back to the local system).
The result is: you are saying to Pi-hole to use itself as upstream.
Seem ok now, but I got this error today: Connection error (**127.0.0.1#5335** ): TCP connection failed while receiving payload length from upstream (**Connection prematurely closed by remote server** )
This error is just saying Pi-hole sent a query to Unbound, waited for a response, Unbound never answers the query back to Pi-hole and closes the connection.
This is an Unbound issue (already reported to unbound).
There is nothing Pi-hole can do about it. There is nothing you can do about it, unless unbound releases a fix.