Having trouble making requests using an access token

Dear support,

I would love your assistance, I feel like I am missing something basic
We have just started working on an integration with the backstage API.
To authorize we use the 'Client Credentials' flow, with client_id and client_secret, and recieve an access_token.
When making any requests with the access_token, e.g. '/users/current/account' the response is a 200 o.k. with a login page, instead of the actual JSON data we want.
Thanks in advance