I had a similar issue to (Shiny apps fail in Microsoft Edge with status 400) with Edge using nginx. I just wanted to drop a note here in case anyone else was having the same issue (as that thread was closed). It was fixed by changing my nginx configuration from
location /apps/ {
proxy_pass http://localhost:3838/;
}
to
location /apps/ {
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
proxy_pass http://localhost:3838/;
}