If you run into the problem in Dynamics AX2012 that a form is displaying a field with the message 'Unretrieved value' and you can neither see or edit the contents of the field, try to stop and restart the AOS service and see if the issue is not resolved. Stopping and starting the AOS will refresh the join and make the field accessible.
Restarting the AX worked for me. :-)
I had the same problem, resetting the AX caches fixed it for me without the need of a AOS restart :)