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Yes, I am experiencing this too. I've just wanted to report before finding your message.

Sorry it took so long to respond.

We're aware of the issue and trying to fix it

I've meant Nginx config ;)

Start to test Nginx:

nginx -t

Try raising the timeout:

location ~ .php$ {
     ......
     fastcgi_read_timeout 300;
     ......
}

;request_terminate_timeout = 0

request_terminate_timeout = 300

service nginx reload

service php-7.4-fpm restart

Thank you Nikolai! I will test this and come back to you.

I am sorry that I am slow to respond. This is a volunteer project that I can only work on 1 day / week on Fridays.

Your suggestion was tested, but the outcome remains the same:

2023/02/17 13:52:00 [error] 14614#14614: *1 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: 88.165.95.156, server: discussion.cosmopoliticalcoop.site, request: "POST /action/group_tools/mail HTTP/2.0", upstream: "fastcgi://unix:/var/run/php/php7.4-fpm.sock:", host: "discussion.cosmopoliticalcoop.site", referrer: "https://discussion.cosmopoliticalcoop.site/groups/mail/107"