Your client uses HTTP basic authentication scheme by default, but your server expects "form" authentication scheme.
Your server is not OAuth 2 compliant, see RFC 6749:
2.3.1. Client Password
Clients in possession of a client password MAY use the HTTP Basic
authentication scheme as defined in [RFC2617] to authenticate with
the authorization server. The client identifier is encoded using the
"application/x-www-form-urlencoded" encoding algorithm per
Appendix B, and the encoded value is used as the username; the client
password is encoded using the same algorithm and used as the
password. The authorization server MUST support the HTTP Basic
authentication scheme for authenticating clients that were issued a
client password.
But you can change the authentication scheme of your client to "form", see OAuth 2 Developers Guide:
clientAuthenticationScheme:
The scheme used by your client to authenticate to the access token endpoint. Suggested values: "http_basic" and "form". Default: "http_basic". See section 2.1 of the OAuth 2 spec.