XMLDocumentCover class does not exist

We recently received a report from the developers that they were getting the following errors in one of their Oracle 9i development environments:

ORA-29540: class oracle/xml/parser/plsql/XMLDocumentCover does not exist
ORA-06512: at "SYS.XMLDOCUMENTCOVER", line 0
ORA-06512: at "SYS.XMLDOM", line 1605
ORA-06512: at "SIEBEL.GET_ACCOUNT_INFO", line 1044
ORA-06512: at line 4

The solution was to re-initialize the oracle XDK as follows

sqlplus / as sysdba