I tried the solution provided there and now I don't get the key errors anymore, but again the pacstrap fails:
error: failed to synchronize all databases (unable to lock database)
==> ERROR: Failed to install packages to new root
I tried to delete the .lock file in /mnt/var directory but I still get this error.
EDIT:
I removed everything from /mnt dirctory and start over. This time I didn't get any error and pacstrap finished successfully.