%d ignored from auth-passwdfile.conf.ext configuration file

[Andrei Petru Mura]

I try to configure dovecot with virtual users. I put my users file in
folder /etc/dovecot/my_domain_name/users.
My auth-passwdfile.conf.ext file looks like this:
passdb {
  driver = passwd-file
  args = username_format=%n /etc/dovecot/%d/users
}

When I try to log in, I get this:
dovecot: auth: Error:
passwd-file(test,some.ip.addr.here,<rQT+ziijGE1tpoAB>):
stat(/etc/dovecot//users) failed: No such file or directory

As you can see, %d isn't interpreted. Why is this happening? Any hints?

Thanks,
Mura Andrei