During the upgrade to 6.4.2 the certificates were renewed for splunkd, but not for splunkweb
"/usr/splunk/etc/auth/ca.pem": certificate renewed
"/usr/splunk/etc/auth/cacert.pem": certificate renewed
"/usr/splunk/etc/auth/server.pem": certificate renewed
Those certs, for splunkweb, doesn't come by default with splunk installation and hence not updated during upgrade. You would've either used a self signed certs OR third party certs, so follow the same process to get new certs and update the same in your web.conf.