The Enterprise Portal in Dynamics AX 4.0 and Dynamics AX 2009 is based on Microsoft SharePoint Technologies. This is for sure nothing new for you. Because of this fact you can use a SharePoint compatible editor like Microsoft SharePoint Desinger to modify WebPart Pages and for example add new WebPart Zones to them.
In order to easily deploy the changed WebPart Pages to other Dynamics AX installations, it is a good idea to import the layout of the WebPart Pages to the Dynamics AX AOT.
Sometimes this import fails with the following error message:
There is not only one reason resulting in the error message. Actually nearly every error situation that happens during import of WebPart Pages to the AOT fails with exact this message.
One reason for this error is when you have created a new WebPart Page in SharePoint Designer, rather than creating the WebPart Page first in the "Enterprise Portal" Document Library using the SharePoint Web Interface.
During the import of a WebPart Page Dynamics AX is extracting information about the file system template the WebPart Page is based on (e. g. "SitesTemplates\AXSITEDEF\default.aspx"). Dynamics AX is requiring this information about the file system template as this is needed during the registration of a new Enterprise Portal Web Site (Dynamics AX is not importing the whole WebPart Page into the AOT but only parts of the page).
SharePoint stores this information in the property "vti_setuppath". In case you created a new WebPart Page using SharePoint Designer this page is not based on any file system template and so the property vti_setuppath is not existing. Since the property does not exist a System.NullReference Exception is raised as soon as Dynamics AX is accessing the property and the import of the page fails. Unfortunately I found no way manually to add / change the property vti_setuppath.
The only advice I can give is to allways create new WebPart Pages using the SharePoint Web Interface first, and then start customizing them using Microsoft SharePoint Designer.
As long as Dynamics AX can still find the information it is looking for, customizing with the SharePoint Designer is OK. What information AX is extracting you can see if you got to "AOT\Web\Web Files\Page Definitions" click at any existing Page Definition with the right Mouse button and choose "Edit".