wix-member-backend.authentication.login returns null

Hi there,
I’m kind of stuck regarding authentication and I can’t figure out what’s going on.
I’m using the login function from the wix-members-backend.authentication module in a jsw file. Basically I’m using this examples from the doc: https://www.wix.com/velo/reference/wix-members-backend/authentication/login
However in my case the login function does not return a session token (it returns null).

// sessionToken is null
sessionToken = await authentication.login(email, password);

Any help would be appreciated.
Thanks!