mqtt ssl双向认证例程报-0x2700错误怎么解决？

版本idf-3.2,example/protocols/mqtt/ssl_mutual_aut例程，客户端认证正常，加人服务器认证后报-0x2700错误。
错误信息：
E (1193558) esp-tls: mbedtls_ssl_handshake returned -0x2700
I (1193558) esp-tls: Failed to verify peer certificate!
I (1193558) esp-tls: verification info:   ! The certificate is signed with an unacceptable key (eg bad curve, RSA too short).

E (1193578) esp-tls: Failed to open new connection
E (1193578) TRANS_SSL: Failed to open a new connection
E (1193588) MQTT_CLIENT: Error transport connect

证书定义：
extern const uint8_t client_cert_pem_start[] asm("_binary_client_crt_start");
extern const uint8_t client_cert_pem_end[] asm("_binary_client_crt_end");
extern const uint8_t client_key_pem_start[] asm("_binary_client_key_start");
extern const uint8_t client_key_pem_end[] asm("_binary_client_key_end");
extern const uint8_t server_cert_pem_start[] asm("_binary_mosquitto_org_crt_start");
extern const uint8_t server_cert_pem_end[] asm("_binary_mosquitto_org_crt_end")
mqtt配置：
    const esp_mqtt_client_config_t mqtt_cfg = {
        .uri = "mqtts://test.mosquitto.org:8884",
        .event_handle = mqtt_event_handler,
        .cert_pem = (const char *)server_cert_pem_start,
        .client_cert_pem = (const char *)client_cert_pem_start,
        .client_key_pem = (const char *)client_key_pem_start,
    };