In order to fix this, you must not refresh the keys manually. Apparently, the user locally changed the key to expire on Aug. 2 and published this expiration date change to the keyserver.
Remove the /etc/pacman.d/gnupg folder and follow the instructions in the Wiki: Pacman troubleshooting - Manjaro to start new.
After I reset the keys, I was able to install today’s testing updates which was not possible before.