I'm not sure the connection between mattermost and Jenkins works, I have not looked at the plugin source. A REST API would not be userless though, it would use http authentication.
The errors I see in the Jenkins logs are :
Credentials cannot be used for NTLM authentication: org.apache.commons.httpclient.UsernamePasswordCredentials
org.apache.commons.httpclient.auth.InvalidCredentialsException: Credentials cannot be used for NTLM authentication: org.apache.commons.httpclient.UsernamePasswordCredentials