To reproduce:

1. Go to "Site administration > Advanced features"
2. Tick "Enable portfolios"
3. Click "Save changes"
4. Go to "Site administration > Plugins > Portfolios > Manage portfolios"
5. Make "File download" "Enabled and visible"
6. Go to a course with a forum discussion.
7. Post a reply to that discussion.
8. After posting your reply, click on the "Export to portfolio" link.
9. Select any of the "Available export formats"
10. Click "Next"
11. Click "Continue" to confirm the file export.
12. Check the contents of the downloaded file.
13. Expected
    • The downloaded file should contain the appropriate content.
14. Actual
    • The downloaded file contains the following error message (e.g. in HTML format):

      <br />
      <b>Fatal error</b>: stored_file::readfile(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition &quot;file_system_filedir&quot; of the object you are trying to operate on was loaded before unserialize() gets called or provide a __autoload() function to load the class definition in <b>/home/jun/moodles/stable_master/moodle/lib/filestorage/stored_file.php</b> on line <b>385</b><br />