pacman-key issue

[Frank Zimmermann <frank.zimmermann.berlin@xxxxxxxxxx>]

Good evening,

I'm having trouble with my pacman-keyring which in the end prevents me 
from installing signed packages. Here's the output:
pacman-key --populate archlinux
==> Appending keys from archlinux.gpg...
==> Locally signing trusted keys in keyring...
 -> Locally signing key FOO
==> ERROR: FOO could not be locally signed.

Any hints?

Thanks Frank