openDocument.jsp error "Callback Function cannot be found"

Has anyone seen the error message "TODO: Callback Function cannot be found" when calling /OpenDocument/opendoc/openDocument.jsp in a web application on XI 3.1.

Tracing the error it appears to be originating from within openDocument.jsp but cannot work out what the error is caused by.

The code works in our XI R2 environment without a problem so perhaps there have been some changes between XI R2 and XI 3.1.

Any help would be greatly appreciated.