When clicking on Proof, instead of the proof pulling up you get the Workfront Proof login window. The problem is the user doesn't know their login credentials for Workfront Proof so they are then stuck.
This seems to only happen when a users computer is not accepting cookies.
If you see this problem here is a link that will help you: https://support.google.com/adsense/answer/35730?hl=en What you are doing here is enabling cookies and cache, which keeps the good cookies and should resolve your problem. You may want to go into the Manage Exceptions area and add app.proofhq.com