The Pacman troubleshooting - Manjaro article tells:
1. Remove old (and possibly broken) keys by entering this command:
sudo rm -r /etc/pacman.d/gnupg
2. Reinstall keyrings including the latest keys:
sudo pacman -Sy gnupg archlinux-keyring manjaro-keyring
The question is:
if to remove all keys in step 1, then in the next step 2 how a package manager could verify any package signature if has no any local keys (at least trusted keys)?
Also at that moment local keyring is not initialized (to make a query of packager keys to).
The issue was meet by 2 people: Unable to update (key issues / corrupted packages) - #12 by saintjules
Also it was mentioned 2.5 years ago: https://archived.forum.manjaro.org/t/how-to-solve-keyring-issues-in-manjaro/4020/4