The problem can be corrected by upgrading the affected package to version 2.5.5-6ubuntu0.8 (for Ubuntu 4.10), or 2.5.8-3ubuntu1.1 (for Ubuntu 5.04). In general, a standard system upgrade is sufficient to effect the necessary changes.
Details follow:
Michael Bhola discovered that errors in the http_access configuration, in particular missing or invalid ACLs, did not cause a fatal error. This could lead to wider access permissions than intended by the administrator.