The failure was in ClassLoader.checkCerts that throws the following
exception:

    if (pcerts != null && !compareCerts(pcerts, certs)) {
        throw new SecurityException("class \""+ name +
             "\"'s signer information does not match signer information of other classes in the same package");
    }

(cherry picked from commit aec2d8af)

Upgrade HtmlUnit to v2.18 (from forked version)
(cherry picked from commit f552d48f)