SLRequest, error 401 for specific user



Hi All,
I am using the iOS SLRequest to communicate with the twitter APIs and it works fine for almost all of my users except one!
He always get the error 401, I’ve tried everything but no matters what, he cannot use my app.
I am puzzled since for everyone else it works fine (so far).

The specific API that returns the error is:

Any idea why it’s happening only to him?
I’m pretty sure his phone is set up correctly, I’ve already asked him to check the authorization, the timezone, and every other twitter app works fine.
Thank you for your help.