You may need to enable or disable EM express on 12c. Do not forget you still can use cent browser for java related screens

Here is the steps and controls

Check related info by using below sqls:

If the below queries outputs bring  "0" than it means 12c is already disabled

select dbms_xdb.getHttpPort() from dual; 
select dbms_xdb_config.getHttpsPort() from dual;

If it bring "1" than run below query for details.

SELECT 'https://'||SYS_CONTEXT('USERENV','SERVER_HOST')||'.'||SYS_CONTEXT('USERENV','DB_DOMAIN')||':'||dbms_xdb_config.gethttpsport()||'/em/' from dual;  

 SELECT 'https://'||SYS_CONTEXT('USERENV','SERVER_HOST')||':'||dbms_xdb_config.gethttpsport()||'/em/' from dual;  

To enable em express with https option run below command;
exec dbms_xdb_config.sethttpsport(5500);

To enable em express with http option run below command;
exec dbms_xdb_config.sethttpport(5550);

Disable EM express, you should set port to "0"
exec dbms_xdb_config.sethttpsport(0); 
exec dbms_xdb_config.sethttpport(0);