Tor Browser 4.5a4 does not cope with certain self signed certificate setups.
On certain affected sites, "Add Exception" does nothing useful, except display the same error page, and disable the "Add Exception" button.
Per GeKo on IRC: "seems Tor Browser can't cope with two self-signed certs in a row. this is a bug"
Test URL: https://hkps.pool.sks-keyservers.net/pks/lookup?op=vindex&search=8A6EC81A