This is a bug we have the same issue I opened an SR 4 months ago and the bug is still with development. They did provide a temporary work around that you can test as well.
Can you try the following work around:
1. Connect as SYSDBA user to target database where export is not working. Create a view with name used in 12c DBs:
2. SQL> create view sys."_BASE_USER" as select * from sys.user$;
3. Grant read access on the new view to all users who need the export function (these will be the users who login into the DB prior export).
4. Retry the export function from EM Console with any of the granted DB users.