Got security error with "Security token does not match. You must be login again..." on 12c EM express

When you login as system on 12c EM Express, you may got the following errors after login.

Basically, this error may happened on you haven't privilege to use EM Express, therefore, you should grant "EM_EXPRESS_BASIC" or "EM_EXPRESS_ALL" role to the specified user. 

In general, "EM_EXPRESS_BASIC" has been granted to "EM_EXPRESS_ALL" by default and the DBA role include "EM_EXPRESS_ALL".  

"EM_EXPRESS_ALL" grants a user all privileges required to perform any action in EM Express.

SQL> column grantee format a20

SQL> select grantee, granted_role from dba_role_privs where granted_role = 'EM_EXPRESS_ALL';

GRANTEE         GRANTED_ROLE

-------------------- --------------

DBA     EM_EXPRESS_ALL

SYS     EM_EXPRESS_ALL

SQL> GRANT EM_EXPRESS_ALL TO SYSTEM;

Grant succeeded.

SQL> select grantee, granted_role from dba_role_privs where granted_role = 'EM_EXPRESS_ALL';

GRANTEE         GRANTED_ROLE

-------------------- --------------

SYSTEM         EM_EXPRESS_ALL

DBA     EM_EXPRESS_ALL

SYS     EM_EXPRESS_ALL

SQL> 

After that, when you login again as system on EM Express, you can successfully to view the database instance performance. If you still got the security errors, please try use another browsers.

Reference:

https://blogs.oracle.com/db/entry/troubleshooting_why_em_express_is