Pi-hole update not working - no connection to update server?

Pi-hole is running nicely. When trying to perform an update via pihole -up, the following is displayed, but no further progress:

Linux raspberrypi 4.14.71-v7+ #1145 SMP Fri Sep 21 15:38:35 BST 2018 armv7l

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
Last login: Wed May 11 07:31:39 2022 from 192.168.1.12
pi@raspberrypi:~ $ pihole -up
  [i] Update local cache of available packages...

The command sudo apt upgrade also does not work, it stops at "0%":

Linux raspberrypi 4.14.71-v7+ #1145 SMP Fri Sep 21 15:38:35 BST 2018 armv7l

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
Last login: Wed May 11 20:27:17 2022 from 192.168.1.12
pi@raspberrypi:~ $ sudo apt upgrade
Paketlisten werden gelesen... Fertig
Abhängigkeitsbaum wird aufgebaut.       
Statusinformationen werden eingelesen.... Fertig
Paketaktualisierung (Upgrade) wird berechnet... Fertig
Die folgenden Pakete werden aktualisiert (Upgrade):
  apt apt-transport-https apt-utils base-files bind9-host bluez-firmware
  ca-certificates cron curl dbus dns-root-data dnsutils e2fslibs e2fsprogs
  file gettext-base git git-man gnupg gnupg-agent gpgv isc-dhcp-client
  isc-dhcp-common klibc-utils libapt-inst2.0 libapt-pkg5.0 libasound2
  libasound2-data libbind9-140 libbsd0 libc-bin libc-dev-bin libc-l10n libc6
  libc6-dbg libc6-dev libcomerr2 libcurl3 libcurl3-gnutls libdbus-1-3
  libdns-export162 libdns162 libdw1 libelf1 libexpat1 libfreetype6
  libfreetype6-dev libgcrypt20 libglib2.0-0 libglib2.0-data libgmp10
  libgnutls30 libgssapi-krb5-2 libhogweed4 libicu57 libidn11 libisc-export160
  libisc160 libisccc140 libisccfg140 libjpeg62-turbo libk5crypto3 libklibc
  libkrb5-3 libkrb5support0 libldap-2.4-2 libldap-common libluajit-5.1-common
  liblwres141 liblz4-1 libmagic-mgc libmagic1 libnettle6 libnghttp2-14
  libp11-kit0 libpam-systemd libperl5.24 libpng-dev libpng-tools libpng16-16
  libpolkit-agent-1-0 libpolkit-backend-1-0 libpolkit-gobject-1-0
  libpython2.7-minimal libpython2.7-stdlib libpython3.5 libpython3.5-minimal
  libpython3.5-stdlib libraspberrypi-bin libraspberrypi-dev libraspberrypi-doc
  libraspberrypi0 libsamplerate0 libsasl2-2 libsasl2-modules
  libsasl2-modules-db libseccomp2 libsqlite3-0 libss2 libssh2-1 libssl1.0.2
  libssl1.1 libsystemd0 libtirpc1 libudev1 libwbclient0 libx11-6 libx11-data
  libxapian30 libxml2 lighttpd locales login luajit multiarch-support
  nfs-common openssh-client openssh-server openssh-sftp-server openssl passwd
  patch perl perl-base perl-modules-5.24 php7.0-cgi php7.0-cli php7.0-common
  php7.0-intl php7.0-json php7.0-opcache php7.0-readline php7.0-sqlite3
  php7.0-xml pi-bluetooth policykit-1 python-apt-common python-rpi.gpio
  python2.7 python2.7-minimal python3-apt python3-six python3-urllib3
  python3.5 python3.5-minimal raspberrypi-bootloader raspberrypi-kernel
  raspberrypi-sys-mods raspi-config raspi-copies-and-fills rsync rsyslog
  samba-common sqlite3 ssh sudo systemd systemd-sysv tar tzdata udev unzip
  usbutils vim-common vim-tiny wget wireless-regdb wpasupplicant xxd zlib1g
  zlib1g-dev
171 aktualisiert, 0 neu installiert, 0 zu entfernen und 0 nicht aktualisiert.
Es müssen 173 MB an Archiven heruntergeladen werden.
Nach dieser Operation werden 8.578 kB Plattenplatz zusätzlich benutzt.
Möchten Sie fortfahren? [J/n] j
0% [Verbindung mit raspbian.raspberrypi.org (2a00:1098:0:80:1000:75:0:3)] [Verbindung mit archive.raspberrypi.org (2a00:1098:84:1e0::3)]

Previously this was working smoothly, what can I do?

This is an OS level issue (e.g. for Raspberry Pi OS, see Updating and Upgrading Raspberry Pi OS).

Consider updating your repositories before upgrading your software, i.e.

sudo apt update  &&  sudo apt full-upgrade

You probably should consider to upgrade your distribution as well, as your kernel dates back to 2018 (probably Stretch, which scheduled LTS-EOL is June 30 2022).

Verify with your OS distribution maintainer whether running a dist-upgrade is recommended, or whether a fresh installation would be preferred.

Thanks for your reply. Both

sudo apt update  &&  sudo apt full-upgrade

end with the same issue, "0%" progress is displayed and nothing happens.

Pi-hole could be involved only if it would block access to raspbian.raspberrypi.org.
However, I am not aware of any blocklists that would contain that domain.

Use Tools | Search Adlists to verify whether your Pi-hole would block it.

Other than that, as mentioned, your observation looks very much like an OS level issue.
You should consider to also consult your OS's support channels for additional advice.

It's running now! Performing all the updates, just took ages to load, although my connection is not the limitation. Anyhow, thanks for your help so far!