While using some of the ZAPI API's you will get a 403 Error though the credentials provided are correct
This error will be thrown when "zephyr" is used in API context for some API's
Example : /rest/zephyr/latest/cycle
Replace the context "zephyr" with "zapi" and check the same API
Example : /rest/zapi/latest/cycle
If you still face same issue after replacing the API context, please submit a case to investigate further.