This is caused by 'Function Name' attribute on the page layout region being NULL but with the XML tag still in the page XML i.e.:
<oa:pageLayout id="PageLayoutRN" amDefName="xxjlp.oracle.apps.fnd.por.econtent.server.EContentAM" windowTitle="Portal Guidance Documents" title="Portal Guidance Documents" controllerClass="xxjlp.oracle.apps.fnd.por.econtent.webui.GuidanceDocSumCO" function="">
Check the page you are forwarding to either has a valid value for 'Function Name' or ensure that this tag is removed from the page XML by clicking the 'Set to default' button on the component pallet toolbar.
This is normally caused when a value was added by the developer then removed by highlighting the value and hitting delete. This action keeps the tag in the XML file. You should use the 'Set to default' button which will remove the tag completely.