lsb_release -a
sudo grep -v '^#\|^$' -R /etc/apt/sources.list*
apt policy lighttpd
apt policy libssl1.1
???
Do you need the nginx web daemon as Pi-hole comes with its own lighttpd daemon ?
Looks like a versions conflict that should be able to fix if choose to run only one web daemon (preferably lighttpd).
Ps. If you want a particular version of a package to be installed, you can grab one from the "Version table" like so:
pi@noads:~ $ apt policy libssl1.1
libssl1.1:
Installed: 1.1.0j-1~deb9u1
Candidate: 1.1.0j-1~deb9u1
Version table:
*** 1.1.0j-1~deb9u1 500
500 http://raspbian.raspberrypi.org/raspbian stretch/main armhf Packages
100 /var/lib/dpkg/status
pi@noads:~ $ sudo apt install libssl1.1=1.1.0j-1~deb9u1
Reading package lists... Done
Building dependency tree
Reading state information... Done
libssl1.1 is already the newest version (1.1.0j-1~deb9u1).
0 upgraded, 0 newly installed, 0 to remove and 6 not upgraded.
EDIT: Last thought if need nginx, i believe you can choose to install Pi-hole without web admin/daemon ... not sure.