Having exactly the same problem on open mediavault.
the I think I have traced the error back to the nginx log of the let’s encrypt docker.
2020/02/26 22:08:28 [error] 390#390: 736 upstream prematurely closed connection while reading upstream, client: XXX.XXX.XXX.XXX, server: XXXXX., request: "GET /s/[FILE]
I don’t know if this part of the nextcloud.subdomain.conf is responsible:
location / {
include /config/nginx/proxy.conf;
resolver 127.0.0.11 valid=30s;
set $upstream_nextcloud nextcloud;
proxy_max_temp_file_size 2048m;
proxy_pass https://$upstream_nextcloud:443;
proxy_max_temp_file_size 2048m; this in particular.
Any insights?