The issue that I’m going to describe in this post is very similar to the one described on SAP note 1437667.
Generally after the portal upgrade some pages that before the upgrade were working normally start to give the following error message "Page not found or not available" together with a red error icon.

What I could verify is that in these cases the right message that should be displayed is “No content available for this navigation node” together with a green information icon.

I noticed that the cause of this issue is that after the upgrade the “DefaultContent” property is pointing to an iView that does not exist (See SAP Note 1437667 for more information).
In order to confirm that this solution applies for your case, please follow the steps bellow:
- Check the value of the “DefaultContent” property on your default framework inner page;
- Open the Default Framework Page being used when the issue is happening (generally the SAP standard one);
- Select the “Desktop Inner page” and click on “Open”;
- Select the “Content Area” iView and click on “Properties”;
- Look for the property “DefaultContent ”;
- Copy the value of the property;
- Check if the pointed iView exists on the portal content;
If the iView does not exist on the given path, then apply the solution bellow:
- Keep the Content Area Properties opened on the PC of the portal;
- Find the actual place of the “DefaultContent Message” iView;
- Generaly it is under the pcd path: pcd:portal_content/com.sap.pct/every_user/general/com.sap.portal.noContentMessage
- Select the iView on the PCD catalog (single mouse click in it);
- Look at the quick info window (bellow the PCD tree) and copy the value of the “ID” field;
- Open the “Content Area” iView properties (you should have it in another tab of you Portal Content editor);
- Substitute the value of the property “DefaultContent” by the value copied from the quick info of the “No Content Message” iView (you may use the value of any other iView that you want to be displayed or leave the field blank);
Generally this change applies after a browser refresh, but sometimes it takes some time (maybe until the portal cache is updated).