To reproduce

• Enable portfolios
• Enable file export portfolio (enabled and visible)
• Create an Assignment, upload a test assignment for a user etc. 
• Show the grading screen
• The submission(s) now say 'Export to portfolio' underneat
• click the option

Error, "sorry the requested file could not be found"

Basic analysis...

It's going wrong in mod/assign/locallib.php in function load_data() around line 9252. A check is made that the requester's user id and the uploader's id (the student) are the same. Which they never will be. The failure throws the Exception. 

If the student views the Assignment, they see the link for their own file and it works fine (as expected)