[Dovecot] dovecot dead but subsys locked: auth process died to early, error 127

I am having trouble starting up dovecot on my RHEL5 server.

When starting the dovecot service, I get:

dovecot dead but subsys locked

Looking into the mail logs, I find:

dovecot: Auth process died too early - shutting down dovecot: child 4165 (auth) returned error 127

Does anybody know how I can fix this?

Thanks in advance.