Update:
I found that when i delete /var/lib/pacman/sync and just use sudo pacman -Syyu then everything is fine. As soon as I open the Updated Pacman GUI and start an update here it will fail. Could this come from some AUR Packages?
Edit:
When I run Pamac update I get the Error again:
pamac update
Preparing…
Synchronizing package databases…
Refreshing core.db…
Refreshing extra.db…
Refreshing community.db…
Refreshing multilib.db…
Error: multilib.db: GPGME error: No data
Error: multilib.db: GPGME error: No data
Error: multilib.db: GPGME error: No data
Error: multilib.db: GPGME error: No data
invalid or corrupted database (PGP signature)
Failed to synchronize databases
Deleting /var/lib/pacman/sync and running Pacman -Syyu fixed it but then I am not able to Update my AUR Packages anymore - is there anything known about that issue or is it temporary?