As far as I understand Database Express is build into the kernel on top of XMLDB which is now standard as well.
Oracle Database 2 Day DBA 12c Release 1 "Oracle Enterprise Manager Database Express (EM Express) is a web-based database management tool that is built inside the Oracle Database." Chapter 3.
So you should simply check if the https port is set correctly and if not set it accordingly:
SQL> select dbms_xdb_config.gethttpsport() from dual;
if empty than set it:
SQL> exec DBMS_XDB_CONFIG.SETHTTPSPORT(5500);
after that everything should work (it did when I tested it ;-)
EM Express in 12c is very different(not just in the UI ) from the pre-12.1 installation as it is based on the APEX. Have a look at the below link to enable it for the database,
You don't have to do it manually as it is part of the base installation. I just did the following two tests:
create a new 12c database without using templates and without database express checkbox set. No additional components (e.g. APEX, etc.) installed.
After the creation I was unable to connect to database express - as expected
But after setting the https port --> Database Express worked:
SQL> exec DBMS_XDB_CONFIG.SETHTTPSPORT(5502);
create a new 11g database again without using templates without any additional components (like JVM, XML, ...) and without Enterprise Manager configuration.
Upgrade the database to 12c
After adding the dispatcher:
alter system set dispatchers="(PROTOCOL=TCP)(SERVICE=WOLFEXXDB)";
setting the HTTPS-Port and bouncing the instance Database Express is running.
IT'S MAGIC ;-)