“Unable to display page” with Oracle 12c Enterprise Manager Database Express

i'm using redhat 6, installed 12.1.0.2 of oracle and created a database.

database (dev12ee1) is a non-container database, there is also an 11g database on the server (dev11ee1)

lsnrctl status shows standard port of 1521 and 5500

lsnrctl status

LSNRCTL for Linux: Version 12.1.0.2.0 - Production on 17-JUN-2015 11:46:54

Copyright (c) 1991, 2014, Oracle.  All rights reserved.

Connecting to (ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for Linux: Version 12.1.0.2.0 - Production
Start Date                02-JUN-2015 12:28:48
Uptime                    14 days 23 hr. 18 min. 6 sec
Trace Level               off
Security                  ON: Local OS Authentication
SNMP                      OFF
Listener Log File         /u01/app12/oracle12/diag/tnslsnr/csya-oradev5/listener/alert/log.xml
Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=csya-oradev5.xxxx-dmz.local)(PORT=1521)))
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcps)(HOST=csya-oradev5.xxxx-dmz.local)(PORT=5500))(Security=(my_wallet_directory=/u01/app12/oracle12/admin/dev12ee1/xdb_wallet))(Presentation=HTTP)(Session=RAW))
Services Summary...
Service "dev11ee1.csya-oradev5" has 1 instance(s).
  Instance "dev11ee1", status READY, has 1 handler(s) for this service...
Service "dev12ee1.csya-oradev5" has 1 instance(s).
  Instance "dev12ee1", status READY, has 1 handler(s) for this service...
The command completed successfully

the port settings in sqlplus look like this

SQL> select dbms_xdb_config.gethttpsport() from dual;

DBMS_XDB_CONFIG.GETHTTPSPORT()
------------------------------
                          5500

SQL>

the local_listener and dispatchers database parameters are both blank (which should be ok as everything is standard and default).

but when i go to the URL

https://csya-oradev5.xxxx-dmz.local:5500/em/

i get "This webpage is not available"