Cannot login with shared secret:
# curl -k https://examplebrand.com/servlet/Sessions\?event=login\&login=user\&secret=secret\&session_id=123 <html><head><title>Error</title></head><body>ERROR:javax.servlet.ServletException: Can't login</body></html>
Wrong shared secret is used.
It is necessary to use the shared secret of brand's owner instead of user's for successful login.
Additional attention should be paid to authorized netowrks (Settings > Security > Setup > Authorized Network) of reseller that owns the brand - those settings are applicable to everyone who logs in its brand.