@kvolari
Is your Portal federated? Are you using windows authentication?
I resolved the issue by adding a user to our Portal site and giving them access to the map services. I made the assumption that the user needing access would be the end user, but that was false. It is actually the account used to setup ArcGIS Server. Also, make sure that account is >= publisher.
I was able to determine which user by identifying that a new account was being created in Portal each time I called the Print Task.
Good luck... This one took me days to figure out 🙂