Oracle 12c - Can not login to Enterprise Manager - XDB Login prompt

I have installed Oracle 12c Enterprise Edition on one of my Windows 7/Linux. 

I can as well login to Oracle Web Enterprise Manager (It runs on https://(hostname):5500/em ) using SYS & SYSTEM passwords without specifying any database. However in case of Web Enterprise Manager the moment I choose a database container (See the screenshot, left side login prompt) despite entering correct userid/password it prompts for yet another weblogin prompt. I am clueless what to enter here.

I tried SYS & SYSTEM passwords but it did not help. During installation, it asked for an additional admin password that too does not work here. I also tried unlocking XDB & Anonymous accounts and restarted Database & listener but it did not help. tnsnames.ora has entries for both ORCL & ORCLPDB.

Does anyone have any idea what this login is for and how to proceed further?

Thanks.

You should enable global port for EM Express to Manage a CDB and the PDBs, execute:
exec dbms_xdb_config.SetGlobalPortEnabled(TRUE);