The warnings have always existed in /var/log/pihole.log. A change to Pi-hole made them more visible by also printing them into /var/log/pihole-FTL.log and showing them on the dashboard.
Brief documentation for these warning can be found here:
Both warnings you are seeing have the same meaning and I think the message is pretty clear. How can we improve it?
Let me explain in more words what is happening here:
You have a device that wants to be called rpi.lan (NPT has webserver) in the network. However, your Pi-hole itself already has this hostname. Hence, it declines that the other device gets this name as there can be no two devices with the same name in the network (which one would you pick if querying for this name?). Pi-hole rightfully warns you about this issue as you may expect the other device to be reachable through this hostname.
Solution: Only one of the devices should have this hostname. If your Pi-hole has a different name and the entry for 127.0.1.1 is still showing the old hostname, the solution will be to fix your Pi-hole's /etc/hosts. Otherwise, you should investigate why the other device tries to register with the hostname your Pi-hole already has.
So which device has which hostname? How do their /etc/hosts 127.0.1.1 entries look like? Whatever tool you may have used might have forgotten to also update the /etc/hosts entry.
Same advise to you. Was your Pi-hole ever called "webserver" before? Is it still? You may have to fix one of the /etc/hosts entries as well.