For those with inquiring minds and who do not have access to MetaLink:

Cause:
------
XDB installation/dictionary built causes some of the underlying XML views to be INVALID. Here the error occurs when dbms_xmlgen tries to access one those invalid views.

Fix:
----
Run the following script that resides under $ORACLE_HOME/rdbms/admin directory after XDB dictionary is built by connecting as user SYS : catmeta.sql (Creates Object Model of Oracle dictionary).

After running this script, start the export of database again.