You are trying to authenticate against the Kaseya WebAPI using domain credentials, but this is not working
The Kaseya WebAPI does not support domain authentication.
Workaround / Solution:
When authenticating against the WebAPI, you must use a native Kaseya account, not a domain account
When you log in to the web interface, and use a domain login, Kaseya kicks off a background script that runs a procedure on the domain controller to validate the login.
When accessing the API, the authentication is done directly against the Kaseya database's own user list and is not able to run the domain authentication procedure.