This probably isn’t of much help to you, but I also have the interface behind nginx and all is well on FF and Chrome.
But, I initially had repeated “Syncthing seems to be down” messages, until I adjusted some buffers and timeouts in my nginx config.
These are the relevant parts of my nginx config:
server {
listen 443 ssl http2;
listen [::]:443 ssl http2;
server_name ...
ssl_certificate ... etc
location / {
add_header Strict-Transport-Security "max-age=31536000";
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_buffers 32 4m;
proxy_busy_buffers_size 25m;
proxy_buffer_size 512k;
proxy_read_timeout 600s;
proxy_send_timeout 600s;
proxy_http_version 1.1;
proxy_max_temp_file_size 0;
proxy_pass http://127.0.0.1:8384/;
}
}