Looks like we may have accidentally dropped support for Debain/Raspbian stretch. Sorry folks!
We're working to fix the issue now.
You should see no problems on Buster, so if you are able to , please feel free to upgrade your distribution. For anyone that can't we should have a fix coming very soon™ for Stretch.. watch this space!
(this is the error that manifests as "version `GLIBC_2.28' not found (required by pihole-FTL)")
Edit: Also Ubuntu 16 and 18 / Centos 7
Edit: fixed