Please disregard, I found that the solution was:
1. Start Chrome
2. Go to settings --> privacy and security and delete cookies for *.keyshot.com
3. Restart Chrome
4. Click "sign in" on KeyShot side to sign in again
5. This time the auth handler worked.
I have a customer who is trying to activate their subscription via signing in to their portal account. They are able to sign in via web browser, but after signing in ("You've signed into KeyShot"), it does not return that sign in information to the KeyShot client, even though they have clicked the dialog to allow the KeyShot Auth Handler to do this. On the KeyShot side, it still shows a "sign in" button and they cannot proceed.
I suppose something is being blocked, but what should I have them check?