Without a debug log is not possible to really understand what is happening and we can only guess.
Did you read all v6 announcements? This is probably a web server conflict.
Pi-hole v5 used lighttpd as the web server. Pi-hole v6 has its own web server embedded in FTL.
If lighttpd is still installed and running, port 80 will be in use, so Pi-hole v6 will try to use port 8080.
If you also have another service using this port, you need to change the web server port to something else.
lighttpd is configured to serve pages only from /admin. The other path (/api) is served by the new web server.
Don't do that.
If you did, now you probably have 3 web servers running: lighttpd, civetweb and pihole-FTL.
This command is not needed any more.
If needed, pihole-FTL will automatically restart ifself after any configuration change.