Having a similar issue updating my FTL to latest version see log below:
[i] Existing PHP installation detected : PHP version 7.0.33-0+deb9u3
[i] Performing unattended setup, no whiptail dialogs will be displayed
[✓] Disk space check
[✗] Update local cache of available packages
Error: Unable to update package cache. Please try "apt-get update"
Unable to complete update, please contact Pi-hole Support
pi@raspberrypi:/ $ sudo apt-get update
Hit:2 http://archive.raspberrypi.org/debian stretch InRelease
Get:1 http://raspbian.raspberrypi.org/raspbian stretch InRelease [15.0 kB]
Ign:3 https://dev2day.de/pms jessie InRelease
Ign:4 https://dev2day.de/pms jessie Release
Ign:5 https://dev2day.de/pms jessie/main armhf Packages
Ign:6 https://dev2day.de/pms jessie/main all Packages
Ign:7 https://dev2day.de/pms jessie/main Translation-en_US
Ign:8 https://dev2day.de/pms jessie/main Translation-en
Ign:9 https://dev2day.de/pms jessie/main Translation-en_US.UTF-8
Get:10 http://raspbian.raspberrypi.org/raspbian stretch/non-free Sources [125 kB]
Ign:5 https://dev2day.de/pms jessie/main armhf Packages
Ign:6 https://dev2day.de/pms jessie/main all Packages
Get:11 http://raspbian.raspberrypi.org/raspbian stretch/main Sources [9,719 kB]
Get:12 http://raspbian.raspberrypi.org/raspbian stretch/contrib Sources [75.2 kB]
Ign:7 https://dev2day.de/pms jessie/main Translation-en_US
Ign:8 https://dev2day.de/pms jessie/main Translation-en
Ign:9 https://dev2day.de/pms jessie/main Translation-en_US.UTF-8
Ign:5 https://dev2day.de/pms jessie/main armhf Packages
Ign:6 https://dev2day.de/pms jessie/main all Packages
Ign:7 https://dev2day.de/pms jessie/main Translation-en_US
Ign:8 https://dev2day.de/pms jessie/main Translation-en
Ign:9 https://dev2day.de/pms jessie/main Translation-en_US.UTF-8
Ign:5 https://dev2day.de/pms jessie/main armhf Packages
Ign:6 https://dev2day.de/pms jessie/main all Packages
Ign:7 https://dev2day.de/pms jessie/main Translation-en_US
Ign:8 https://dev2day.de/pms jessie/main Translation-en
Ign:9 https://dev2day.de/pms jessie/main Translation-en_US.UTF-8
Ign:5 https://dev2day.de/pms jessie/main armhf Packages
Ign:6 https://dev2day.de/pms jessie/main all Packages
Ign:7 https://dev2day.de/pms jessie/main Translation-en_US
Ign:8 https://dev2day.de/pms jessie/main Translation-en
Ign:9 https://dev2day.de/pms jessie/main Translation-en_US.UTF-8
Err:5 https://dev2day.de/pms jessie/main armhf Packages
404 Not Found
Ign:6 https://dev2day.de/pms jessie/main all Packages
Ign:7 https://dev2day.de/pms jessie/main Translation-en_US
Ign:8 https://dev2day.de/pms jessie/main Translation-en
Ign:9 https://dev2day.de/pms jessie/main Translation-en_US.UTF-8
Fetched 15.0 kB in 20s (739 B/s)
Reading package lists... Done
W: The repository 'https://dev2day.de/pms jessie Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://raspbian.raspberrypi.org/raspbian/dists/stretch/InRelease Unable to find expected entry 'rPi/source/Sources' in Release file (Wrong sources.list entry or malformed file)
E: Failed to fetch https://dev2day.de/pms/dists/jessie/main/binary-armhf/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.