I Get "401 - Unauthorized" when I try to access the REST API with an access token

I am trying to make a GET User Stories request using the endpoint

https://{my company}.tpondemand.com/api/v1/UserStories/?access_token={my access token}

but I always end up getting a "401-Unauthorized" reponse.

I have already set a System User password, as described in this documentation :