Some users get the error "ERR_EMPTY_RESPONSE" in their browser

Hello,

Some users get the error “ERR_EMPTY_RESPONSE” in their browser.
When I look in the nginx accesslog I see these uses come in with HTTP/1.1 instead of HTTP/2

The vhost is generated like this:
server {
listen 80;
listen [::]:80;

}
server {
listen 4444 ssl http2;
listen [::]:4444 ssl http2;

}

How is this possible? Can it be that http/2 does not work for some users?
Thank you in advance