I had a look at this with our team and it looks like there was a quirk happening with the flow that specifically happens when a user is already logged in to Constant Contact and initiates the OAuth flow. After looking at this with the team that owns the affected application, we were able to find the cause and should be able to have the fix in place in the next couple of weeks.
For the immediate term, the solution to this problem is to simply try the flow again, as you should already be logged out of your account and see success on the second try. It's also worth noting that this does not impact all versions of Chrome, as I am not personally able to reproduce the problem on Chrome 44.0.2403.155.
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