For me the https://api.linkedin.com/v1/people/~?format=json&oauth2_access_token=[accessToken] didn’t work.
Adding reguest header
Authorization: Bearer [accessToken]
didn’t work until I went through linked in sdk and tested and found out that they also require you to add request header
x-li-src: msdk
With those two headers the call https://api.linkedin.com/v1/people/~?format=json worked.
Hopefully Linkedin fixes this undocumented requirement soon…