There are two ways primary ways of saving documents into NetDocuments, ndOffice and ndMail.
This error message is referring to using ndOffice, not ndMail.
I can reliable create this error message when I save an email into NetDocuments via ndSave(ndOffice) and the immediately delete or move the message after I click ndSave.
The interesting thing is that everything time I have recreated this message, the email still gets saved into NetDocuments.
If you wait a couple of seconds before you move the email, the error message should stop. If it does not, I would contact NetDocuments Support: 801-226-6882, Option 2