pacman -Syu seems to work normally. It does not update the multilib.db, so it could be due to that.
pamac update --force-refresh results in the same error.
It seems to be a problem with some gpg keys so I also tried to refresh the keys as mentioned here, which did not resolve the problem
>>> pamac update --force-refresh
Preparing...
Synchronisiere Paketdatenbanken...
Aktualisierung von core.db...
Aktualisierung von extra.db...
Aktualisierung von community.db...
Aktualisierung von multilib.db...
Aktualisierung von core.files...
Aktualisierung von extra.files...
Aktualisierung von community.files...
Aktualisierung von multilib.files...
Fehler: multilib.files: GPGME-Fehler: Keine Daten
Refreshing AUR...
Resolving dependencies...
Checking inter-conflicts...
sudo pacman -Syu [1]
:: Synchronizing package databases...
core is up to date
extra is up to date
community 7.6 MiB 12.9 MiB/s 00:01 [#####################################################] 100%
multilib is up to date
:: Starting full system upgrade...
resolving dependencies...
looking for conflicting packages...