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This is not related to WordPress. It's related to either Apache or Nginx using the HTTP/2 standard.

REFUSED_STREAM (0x7): The endpoint refused the stream prior to performing any application processing (see Section 8.1.4 for details).

It can either come from too many concurrent streams:

Endpoints MUST NOT exceed the limit set by their peer. An endpoint that receives a HEADERS frame that causes its advertised concurrent stream limit to be exceeded MUST treat this as a stream error (Section 5.4.2) of type PROTOCOL_ERROR or REFUSED_STREAM. The choice of error code determines whether the endpoint wishes to enable automatic retry (see Section 8.1.4) for details).

It can also be sent during a Push Response operation:

If the client determines, for any reason, that it does not wish to receive the pushed response from the server or if the server takes too long to begin sending the promised response, the client can send a RST_STREAM frame, using either the CANCEL or REFUSED_STREAM code and referencing the pushed stream's identifier.

Or if the client is trying to connect using HTTP/1.1:

Servers that don’t wish to process the HTTP/1.1 response should reject stream 1 with a REFUSED_STREAM error code immediately after sending the connection preface to encourage the client to retry the request over the upgraded HTTP/2 connection.

There is no way for me to pinpoint what is happening during those requests, as it can have multiple reasons, as stated above.

So I suggest you a couple of options:

• Pass your site's traffic throughugh Cloudflare, so they act as a middle-man for these connections and normalize the requests sent to your server
• You can increase the SETTINGS_MAX_CONCURRENT_STREAMS to minimize the risk of sending a REFUSED_STREAM. If you use Nginx, you can see how to do this here: http://nginx.org/en/docs/http/ngx_http_v2_module.html#http2_max_concurrent_streams
• If you don't know how to do the above, contact your hosting company and ask them to do it for you and upgrade your Nginx version, as some older versions are known to have issues.
• Disable HTTP/2 in Nginx. How to disable http2 in nginx
• As a last resource, you can migrate to another hosting company.

If you use Apache, everything I said above applies to it as well.