Are you get this error while using the updatelistitems method in SHAREPOINT web service (list.asmx)?
Are you aiming to data stored in a subsite?
If so, the solution is to point the webservice of the subsite directly.
If your main site is http://cia, and your subsite is http://cia/coa, you should point to the webservice:
http://cia/coa/_vti_bin/list.asmx
Pay attention that, sometimes, when you update the webreference, in the app.config it remains the same. So change it there as well.
Good luck!
