Problem Authenticating by NTLM

We are trying to specify NTLM authentication for a portlet, to allow the user to navigate to this portlet without the need to enter his user and password. We already set the domain server to use in Folders > Administrative Folders > Administration Dashboard > Configuration > NTLM Authentication Administration and we set the authentication type for the portlet to NTLM. When we try to see the portlet we get the following error: “Failed to negotiate”. We checked if the domain server name was incorrect, but when we change it to an incorrect name for testing purposes we got a different error, so the problem must be something else. (webMethods 7.1.2)