If you've tried to upgrade pi-hole running on 18.04LTS then you might have gotten an error like this:
The following packages have unmet dependencies: lighttpd : Depends: libssl1.1 (>= 1.1.0) but it is not going to be installed E: Unable to correct problems, you have held broken packages.
I've spent a bit of time poking around at this to no avail, until I stumbled upon a solution that is - install aptitude and use it to fix the dependency issue!
root@domains:~# apt install aptitude
root@domains:~# aptitude install lighttpd
and choose downgrade libss1.1
and lighttpd installs and pi-hole works again.
This was a much harder solution to find than it should have been and something is really screwy with these packages.
Hope this helps!