Rockin, still a no go. Sure hope this doesn't send you on a wild goose chase then turn out a configuration issue on this end.
Here's an excerpt from the nginx error log after an upload:
2022/10/21 00:23:10 [error] 2571542#2571542: *26 upstream sent invalid "Content-Length" header: "Content-Length: " while reading response header from upstream, client: IP addy, server: bbs.site.com, request: "GET /content?action=dlattach;topic=139214.0;attach=254415;image HTTP/2.0", upstream: "fastcgi://unix:/run/php-fpm/site.sock:", host: "bbs.site.com", referrer: "https://bbs.site.com/content?topic=139214.msg1369697"
2022/10/21 00:23:10 [error] 2571542#2571542: *26 upstream sent invalid "Content-Length" header: "Content-Length: " while reading response header from upstream, client: IP addy, server: bbs.site.com, request: "GET /content?action=dlattach;topic=139214.0;attach=254417;image HTTP/2.0", upstream: "fastcgi://unix:/run/php-fpm/site.sock:", host: "bbs.site.com", referrer: "https://bbs.site.com/content?topic=139214.msg1369697"
2022/10/21 00:23:10 [error] 2571542#2571542: *26 upstream sent invalid "Content-Length" header: "Content-Length: " while reading response header from upstream, client: IP addy, server: bbs.site.com, request: "GET /content?action=dlattach;topic=139214.0;attach=254419;image HTTP/2.0", upstream: "fastcgi://unix:/run/php-fpm/site.sock:", host: "bbs.site.com", referrer: "https://bbs.site.com/content?topic=139214.msg1369697"
2022/10/21 00:23:10 [error] 2571542#2571542: *26 upstream sent invalid "Content-Length" header: "Content-Length: " while reading response header from upstream, client: IP addy, server: bbs.site.com, request: "GET /content?action=dlattach;topic=139214.0;attach=254421;image HTTP/2.0", upstream: "fastcgi://unix:/run/php-fpm/site.sock:", host: "bbs.site.com", referrer: "https://bbs.site.com/content?topic=139214.msg1369697"
2022/10/21 00:23:10 [error] 2571542#2571542: *26 upstream sent invalid "Content-Length" header: "Content-Length: " while reading response header from upstream, client: IP addy, server: bbs.site.com, request: "GET /content?action=dlattach;topic=139214.0;attach=254423;image HTTP/2.0", upstream: "fastcgi://unix:/run/php-fpm/site.sock:", host: "bbs.site.com", referrer: "https://bbs.site.com/content?topic=139214.msg1369697"
2022/10/21 00:23:10 [error] 2571542#2571542: *26 upstream sent invalid "Content-Length" header: "Content-Length: " while reading response header from upstream, client: IP addy, server: bbs.site.com, request: "GET /content?action=dlattach;topic=139214.0;attach=254425;image HTTP/2.0", upstream: "fastcgi://unix:/run/php-fpm/site.sock:", host: "bbs.site.com", referrer: "https://bbs.site.com/content?topic=139214.msg1369697"
2022/10/21 00:23:10 [error] 2571542#2571542: *26 upstream sent invalid "Content-Length" header: "Content-Length: " while reading response header from upstream, client: IP addy, server: bbs.site.com, request: "GET /content?action=dlattach;topic=139214.0;attach=254427;image HTTP/2.0", upstream: "fastcgi://unix:/run/php-fpm/site.sock:", host: "bbs.site.com", referrer: "https://bbs.site.com/content?topic=139214.msg1369697"
2022/10/21 00:23:10 [error] 2571542#2571542: *26 upstream sent invalid "Content-Length" header: "Content-Length: " while reading response header from upstream, client: IP addy, server: bbs.site.com, request: "GET /content?action=dlattach;topic=139214.0;attach=254429;image HTTP/2.0", upstream: "fastcgi://unix:/run/php-fpm/site.sock:", host: "bbs.site.com", referrer: "https://bbs.site.com/content?topic=139214.msg1369697"
2022/10/21 00:23:10 [error] 2571542#2571542: *26 upstream sent invalid "Content-Length" header: "Content-Length: " while reading response header from upstream, client: IP addy, server: bbs.site.com, request: "GET /content?action=dlattach;topic=139214.0;attach=254431;image HTTP/2.0", upstream: "fastcgi://unix:/run/php-fpm/site.sock:", host: "bbs.site.com", referrer: "https://bbs.site.com/content?topic=139214.msg1369697"
2022/10/21 00:23:10 [error] 2571542#2571542: *26 upstream sent invalid "Content-Length" header: "Content-Length: " while reading response header from upstream, client: IP addy, server: bbs.site.com, request: "GET /content?action=dlattach;topic=139214.0;attach=254433;image HTTP/2.0", upstream: "fastcgi://unix:/run/php-fpm/site.sock:", host: "bbs.site.com", referrer: "https://bbs.site.com/content?topic=139214.msg1369697"
2022/10/21 00:23:10 [error] 2571542#2571542: *26 upstream sent invalid "Content-Length" header: "Content-Length: " while reading response header from upstream, client: IP addy, server: bbs.site.com, request: "GET /content?action=dlattach;topic=139214.0;attach=254435;image HTTP/2.0", upstream: "fastcgi://unix:/run/php-fpm/site.sock:", host: "bbs.site.com", referrer: "https://bbs.site.com/content?topic=139214.msg1369697"
2022/10/21 00:23:25 [error] 2571542#2571542: *26 upstream sent invalid "Content-Length" header: "Content-Length: " while reading response header from upstream, client: IP addy, server: bbs.site.com, request: "GET /content?action=dlattach;sa=tmpattach;attach=post_tmp_777_6e976dbbe41ba5426878a1e07a75209b;topic=139214 HTTP/2.0", upstream: "fastcgi://unix:/run/php-fpm/site.sock:", host: "bbs.site.com", referrer: "https://bbs.site.com/content?action=post;topic=139214.0;last_msg=1369694"
And access log from the same timeframe:
IP addy - - [21/Oct/2022:00:23:10 +0000] "GET /content?action=dlattach;topic=139214.0;attach=254415;image HTTP/2.0" 502 552 "https://bbs.site.com/content?topic=139214.msg1369697" "Mozilla/5.0 (Linux; Android 12; SM-G998U) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/106.0.0.0 Mobile Safari/537.36"
IP addy - - [21/Oct/2022:00:23:10 +0000] "GET /content?action=dlattach;topic=139214.0;attach=254417;image HTTP/2.0" 502 552 "https://bbs.site.com/content?topic=139214.msg1369697" "Mozilla/5.0 (Linux; Android 12; SM-G998U) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/106.0.0.0 Mobile Safari/537.36"
IP addy - - [21/Oct/2022:00:23:10 +0000] "GET /content?action=dlattach;topic=139214.0;attach=254419;image HTTP/2.0" 502 552 "https://bbs.site.com/content?topic=139214.msg1369697" "Mozilla/5.0 (Linux; Android 12; SM-G998U) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/106.0.0.0 Mobile Safari/537.36"
IP addy - - [21/Oct/2022:00:23:10 +0000] "GET /content?action=dlattach;topic=139214.0;attach=254421;image HTTP/2.0" 502 552 "https://bbs.site.com/content?topic=139214.msg1369697" "Mozilla/5.0 (Linux; Android 12; SM-G998U) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/106.0.0.0 Mobile Safari/537.36"
IP addy - - [21/Oct/2022:00:23:10 +0000] "GET /content?action=dlattach;topic=139214.0;attach=254423;image HTTP/2.0" 502 552 "https://bbs.site.com/content?topic=139214.msg1369697" "Mozilla/5.0 (Linux; Android 12; SM-G998U) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/106.0.0.0 Mobile Safari/537.36"
IP addy - - [21/Oct/2022:00:23:10 +0000] "GET /content?action=dlattach;topic=139214.0;attach=254425;image HTTP/2.0" 502 552 "https://bbs.site.com/content?topic=139214.msg1369697" "Mozilla/5.0 (Linux; Android 12; SM-G998U) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/106.0.0.0 Mobile Safari/537.36"
IP addy - - [21/Oct/2022:00:23:10 +0000] "GET /content?action=dlattach;topic=139214.0;attach=254427;image HTTP/2.0" 502 552 "https://bbs.site.com/content?topic=139214.msg1369697" "Mozilla/5.0 (Linux; Android 12; SM-G998U) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/106.0.0.0 Mobile Safari/537.36"
IP addy - - [21/Oct/2022:00:23:10 +0000] "GET /content?action=dlattach;topic=139214.0;attach=254429;image HTTP/2.0" 502 552 "https://bbs.site.com/content?topic=139214.msg1369697" "Mozilla/5.0 (Linux; Android 12; SM-G998U) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/106.0.0.0 Mobile Safari/537.36"
IP addy - - [21/Oct/2022:00:23:10 +0000] "GET /content?action=dlattach;topic=139214.0;attach=254431;image HTTP/2.0" 502 552 "https://bbs.site.com/content?topic=139214.msg1369697" "Mozilla/5.0 (Linux; Android 12; SM-G998U) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/106.0.0.0 Mobile Safari/537.36"
IP addy - - [21/Oct/2022:00:23:10 +0000] "GET /content?action=dlattach;topic=139214.0;attach=254433;image HTTP/2.0" 502 552 "https://bbs.site.com/content?topic=139214.msg1369697" "Mozilla/5.0 (Linux; Android 12; SM-G998U) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/106.0.0.0 Mobile Safari/537.36"
IP addy - - [21/Oct/2022:00:23:10 +0000] "GET /content?action=dlattach;topic=139214.0;attach=254435;image HTTP/2.0" 502 552 "https://bbs.site.com/content?topic=139214.msg1369697" "Mozilla/5.0 (Linux; Android 12; SM-G998U) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/106.0.0.0 Mobile Safari/537.36"