I know it's not any help to you right now, but I asked a while back that they give us more information on account linking errors. It really is a complete black box, once we send the user off, and if something goes wrong there's no way to troubleshoot other than via trial and error. They seemed very receptive to the idea, so maybe we'll see something change soon.
Yes, more information would be great. There's some negative feedback on one of my skills that suggest that account linking doesn't always work, though I have no idea why. I've tested all the devices I have (browsers, ipads, android devices) and it always works. But I don't doubt the reviewers who say it doesn't. One thing to check is whether there's any of your logic during linking that's causing the problem, e.g., calls to non-https sites. Even something like a call to google.apis for fonts....that can cause trouble. Steve