When using Active Directory authentication, you may see an authenticate error if you have a multi-level Forest.
"Authentication failed due to invalid credentials"
The REST API service is unable to traverse the Forest through direct referrals on the LDAP port. You need to use Global Catalog to query the entire Forest.
If you were using the port 389 change it to 3268, if you were using the port 636 change it to 3269.