"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&gt; select dbms_xdb_config.gethttpsport() from dual;

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

SQL&gt;


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"
Try browsing to
https://[IP-address]/em  

For me this works, but everything is -very- -very- slow (a few minutes just to load the login page).

I'm suspecting the slowness has something to do with the certificate. Why the IP-address works and the hostname doesn't... probably some configuration issue.  

At least now anyone with more experience has something to go on...

For reference, all these posts seem to be about the same issue to me, but so far none has been answered:  


Unable to open/connect to Oracle 12c Enterprise Manager Express
Start Oracle 12c enterprise manager in Windows 7
How to run ORACLE 12C EM?
Oracle 12c Express Enterprise Manager on Centos 6.5 is not reachable