Inconsistent ERR_HTTP2_SERVER_REFUSED_STREAM error on page-load

I do have at least two wordpress sites which very inconsistently throw a varying number of net::ERR_HTTP2_SERVER_REFUSED_STREAM errors. When these errors occur the number of errors thrown highly varies from page-load to page-load (or reload) from say 4 requests with that error to about 60 and sometimes even more (if the page has some many requests). The actually affected ressources/requests seem completly random and therefore don't leave any clue where this is coming from.

If these errors occur their occurrence mostly persists (when doing a simple page refresh or hard refresh) until the browser is restarted. Seldomly they even stay after a restart as well.

When this hiccup does happen and the browser/system gets in this faulty state, these errors also happen in the wordpress backend loading basic files like .../wp-includes/js/wp-lists.min.js?ver=5.7 and similar.

At least two users have experienced this behaviour in Chrome, Opera and Edge while being logged-in to and -out of wordpress. In Opera and Edge we do not have any browser extensions installed. As far as we know other users never had this issue even though some of them visited the site many times.

What might be the reasons for this and/or what might be a way to solve it?

List of Plugins installed on both sites:

• Image Map Pro
• Kadence Blocks - PRO Extension
• Kadence Blocks – Gutenberg Blocks for Page Builder Features
• Kadence Galleries
• Kadence Pro
• Lessify Wordpress
• Password Protect WordPress Lite
• Presto Player
• Presto Player Pro
• Slider Revolution
• WP Google Maps
• WP Google Maps - Pro Add-on
• WPvivid Backup Plugin
• WPvivid Backup Pro

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 through 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.