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The LinkedIn activation guide is missing a step to add the login service to the App when creating a new link between WPDiscuz and LinkedIn.
It’s located in the 3rd tab on LinkedIn Developer site (App Page), if this is not created you are unable to login using the API.
I spent 2 hours then suddenly realised why the oAuth fails with a red error on Wordpress.
Hopefully this will be helpful to others struggling with the same issue.