ES4 Assembler and portfolios' folders compatibility with Acrobat 9

Question

I have the following problem: I'm producing a PDF Portfolio with some nested file and folders through Assembler WebAPI.
The portfolio is ok and works fine if opened with Acrobat XI, but the same portfolio is bugged if opened with Acrobat 9: all the package files are shown in the root directory and clicking (or right-clicking) on them causes a navigation to the (empty) directory specified in the DDX. There is also no way to open the package files.
(PDF Portfolios created with Acrobat 9 works fine in both versions, so isn't a problem related to my installation of Acrobat 9)
How can i make it works also in Acrobat 9? I'm defining a wrong DDX or it is a known bug?
The file should be distributed to clients who use Acrobat 9 and cannot install newer version due to company policies.
Example DDX (tried also many other forms, like using PDF source elements or defining a PackageFiles element with a file:/// source including subfolders, but the results in Acrobat 9 is always bugged):
<DDX xmlns="http://ns.adobe.com/DDX/1.0/">
  <PDF result="output">
    <Portfolio>
      <Navigator source="MyNavigator" />
      <ColorScheme scheme="blueScheme" />
    </Portfolio>
    <Folder name="Folder1">
      <Folder name="Subfolder1">
        <PackageFiles source="source1.pdf" nameKey="source1.pdf">
          <File filename="source1.pdf" mimetype="application/pdf" />
        </PackageFiles>
        <PackageFiles source="source2.pdf" nameKey="source2.pdf">
          <File filename="source2.pdf" mimetype="application/pdf" />
        </PackageFiles>
      </Folder>
    </Folder>
  </PDF>
</DDX>
Thanks.

Answer 1

I got the error msg translated:
Cannot read input data, some characters cannot be showed/printed.
I ran preflight and embedded the fonts and I'm hoping that fixes the problem.  I haev no way of testing this.  Does anyone know if I did the right thing or is there something else I should do?