I had created a channel for a LinkedIn company page. Worked perfectly, like the others. Of a sudden it stopped working, while actually nothing has changed. In the backend I am now getting a 500 error (no refresh token is available).
When I try to create a new LinkedIn channel, all goes well, until I slect the user, I get the same 500-error.
Can you go into your application on developer.linkedin.com and click on the Regenerate button to generate another user token? Log in again via LinkedIn from your front-end and then try to refresh/edit the channel again to see what happens. I've personally had random behavior similar to what you're describing for LinkedIn and that has helped me before. Let me know if there's any change.