现在需要核对2018年的数据，使用的是开发文档V2版本，下载资金账单接口返回[商户证书不合法]，但是用同样的商户号同样的证书走退款流程却没有问题。请求参数和返回响应信息如下：

-->request start

{

url: https://api.mch.weixin.qq.com/pay/downloadfundflow,

method: POST,

headers: [

ACCEPT: */*,

Content-Type: text/xml;charset=UTF-8,

Content-Length: 236,

Host: api.mch.weixin.qq.com,

Connection: Keep-Alive,

Accept-Encoding: gzip,

User-Agent: okhttp/3.3.0

],

body: <xml><nonce_str>d1d72a70906d4afebe0b824238849ce2</nonce_str><appid>xxxx</appid><mch_id>xxxx</mch_id><sign>xxxx</sign><bill_date>20180101</bill_date><account_type>Basic</account_type></xml>,

protocol: http/1.1

}

-->request end 

2020-12-14 18:26:20.821 +0800 [payment-api][DEV][INFO] from com.xingren.service.client.interceptors.HttpResponseLogInterceptor in Test worker - [vclient]{} 

<--response start

{

code: 200,

msg: OK,

url: https://api.mch.weixin.qq.com/pay/downloadfundflow,

headers: [

Server: nginx,

Date: Mon, 14 Dec 2020 10:26:20 GMT,

Content-Type: text/plain; charset=utf-8,

Content-Length: 114,

Connection: keep-alive,

Keep-Alive: timeout=8

],

body: <xml><return_code><![CDATA[FAIL]]></return_code><return_msg><![CDATA[商户证书不合法]]></return_msg></xml>,

tookMs: 233ms

}

有大佬帮忙解决一下嘛？签名已经用文档里的微信支付接口签名校验工具验证过了，没问题。