Forgot to run END_UPGRADE during a Timezone File Upgrade (ORA-56924)
Just start the database normally and run DBMS_DST.UPGRADE_DATABASE

Then you  must see the messages "An upgrade window has been successfully ended." and "Failures:0"

Double check that it succeeded  by running:

1: 

SELECT PROPERTY_NAME, SUBSTR(property_value, 1, 30) value
    FROM DATABASE_PROPERTIES
    WHERE PROPERTY_NAME LIKE 'DST_%'
    ORDER BY PROPERTY_NAME;


2:

 SELECT * FROM v$timezone_file;


-- file with the newer version must appear here.

Lastly, make sure the versions match for the following queries:

CONN / as sysdba
select VERSION from v$timezone_file;
select TZ_VERSION from registry$database;


--if they differ after an upgrade then updating registry$database can be done by

CONN / as sysdba

update registry$database set TZ_VERSION = (select version FROM v$timezone_file);
commit;


Hope it helps.