Link: http://www.mqaa.nl/2009/06/19/oracle-xe-and-utl_http/
This was working OK on our database server but my local Oracle XE instance gave the following error:
ORA-00904: invalid identifier
After looking around a bit I found that the utl_http library should be available in XE. So after digging a bit further I found the reason why it was not working. The fix is to login as sys (not system!) and execute the following command:
grant execute on utl_http to public
After this all is working OK again