I have already found an error in the site engine code. The problem was that
I did not expect that the CMC developers would make a gross mistake - they
would give data before the header() function.
Posted at Nginx Forum:
https://forum.nginx.org/read.php?2,291303,291309#msg-291309
I have a bunch of Nginx + PHP8.0-fpm installed in the north. When crawlers
visit a page and the request contains the If-Modified-Since header, but the
page has not changed, the site returns the "304 Not Modified" header. If the
request protocol is HTTP / 2.0, everything works correctly, but if the
