ssl_trusted_certificate should point to chain.pem if we're demonstrating LetsEncrypt: https://community.letsencrypt.org/t/howto-ocsp-stapling-for-nginx/13611/5

installation/pleroma.nginx

@@ -31,7 +31,7 @@ server {
     listen 443 ssl http2;
     ssl_session_timeout 5m;
 
-    ssl_trusted_certificate   /etc/letsencrypt/live/example.tld/fullchain.pem;
+    ssl_trusted_certificate   /etc/letsencrypt/live/example.tld/chain.pem;
     ssl_certificate           /etc/letsencrypt/live/example.tld/fullchain.pem;
     ssl_certificate_key       /etc/letsencrypt/live/example.tld/privkey.pem;