This is a continuing discussion about the article Making requests to the Zendesk API in the developer documentation.
Making requests to the Zendesk API
Login to the community
No account yet? Create an account
Enter your E-mail address. We'll send you an e-mail with instructions to reset your password.

Hi Tomasz -- since this is a sandbox instance, users (and related info such as passwords, etc) would need to be recreated in the sandbox. Sandboxes are completely separate instances than the "production" instance. Perhaps this particular user and password are different in the sandbox?? Just guessing here.
If the user and password are accurate for the sandbox instance, please submit a ticket to support@zendesk.com. It may require more details and access to your account that just can't be done over a community conversation. Thanks!