@Benjamin - Same issue here at the exact same step in setting up OAuth. I almost gave up when I came across a simliar setup in the SmartThings Alfred Web Service.
It looks like the Authorize and Token requests are passing along a scope=app parameter which appears to have done the trick when I tried it on my end.
When you are requesting the Access Grant from /oauth/authorize, try including a parameter of scope=app and then include this scope=app parameter again when fetching the access token from /oauth/token.
Hope this helps!