Oracle 12c Express Enterprise Manager on Centos 6.5 is not reachable
0> "and set 5500 was impossible due to the error - 5500 seemed busy"

This means that the port 5500 is already "in use". Well, I bet it is not in use but Oracle thinks that. There are a lot of ports reserved in /etc/services.

Open the file and change the following lines:

fcp-addr-srvr1  5500/tcp                # fcp-addr-srvr1
fcp-addr-srvr1  5500/udp                # fcp-addr-srvr1


Change them into:

#fcp-addr-srvr1  5500/tcp                # fcp-addr-srvr1
#fcp-addr-srvr1  5500/udp                # fcp-addr-srvr1


Retry to set the port with SQL&gt; EXEC dbms_xdb_config.sethttpport(5500);.



EDIT 1:

Your listener shows that it's listening on port 5500 for https requests. So maybe you already did what I just posted.
Could you please share the output of the following command?

service iptables status




EDIT 2:

I just activated EM Express on one of my 12c databases. Here is what I did:


OS: Oracle Linux 6.5
DB: Oracle Database 12.1.0.2.0
service iptables status --> OFF


Action log:

$ sqlplus / as sysdba
SQL&gt; SELECT DBMS_XDB_CONFIG.gethttpport FROM dual;

GETHTTPSPORT
------------
        5500

SQL&gt; SELECT DBMS_XDB_CONFIG.gethttpsport FROM dual;

GETHTTPPORT
-----------
          0


I'm going to set https port to 5501 and overwrite the http port with 5500.

SQL&gt; EXEC DBMS_XDB_CONFIG.sethttpport(5500);
PL/SQL procedure successfully completed.

SQL&gt; EXEC DBMS_XDB_CONFIG.sethttpsport(5501);
PL/SQL procedure successfully completed.


Afterwards the listener shows that I's listening on these ports (before the 5500 record was there already but it was not working)

$ lsnrctl status | grep PORT
Connecting to (ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521))
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=***)(PORT=1521)))
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcps)(HOST=***)(PORT=5501))(Security=(my_wallet_directory=/opt/oracle/admin/orcl/xdb_wallet))(Presentation=HTTP)(Session=RAW))
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=***)(PORT=5500))(Presentation=HTTP)(Session=RAW))


I did the test in a browser:


Browse to http://***:5500/em --> login screen appears
Browse to https://***:5500/em --> login screen appears


Note the tailing "/em" is important.