Enable http/2 for the panel and Apps vhost.

install/tpl/nginx_apps.vhost.master

 server {
-        listen {apps_vhost_ip}{apps_vhost_port} {ssl_on};
-        listen [::]:{apps_vhost_port} {ssl_on} ipv6only=on;
+        listen {apps_vhost_ip}{apps_vhost_port} {ssl_on} http2;
+        listen [::]:{apps_vhost_port} {ssl_on} ipv6only=on http2;
 
         {ssl_comment}ssl_protocols TLSv1.2;
         {ssl_comment}ssl_certificate /usr/local/ispconfig/interface/ssl/ispserver.crt;

install/tpl/nginx_ispconfig.vhost.master

 server {
-        listen {vhost_port} {ssl_on};
-        listen [::]:{vhost_port} {ssl_on} ipv6only=on;
+        listen {vhost_port} {ssl_on} http2;
+        listen [::]:{vhost_port} {ssl_on} ipv6only=on http2;
 
 		{ssl_comment}ssl_protocols TLSv1.2;
         {ssl_comment}ssl_certificate /usr/local/ispconfig/interface/ssl/ispserver.crt;

server/conf/nginx_apps.vhost.master

 server {
-        listen {apps_vhost_ip}{apps_vhost_port} {ssl_on};
-        listen [::]:{apps_vhost_port} {ssl_on} ipv6only=on;
+        listen {apps_vhost_ip}{apps_vhost_port} {ssl_on} http2;
+        listen [::]:{apps_vhost_port} {ssl_on} ipv6only=on http2;
 
         {ssl_comment}ssl_protocols TLSv1.2;
         {ssl_comment}ssl_certificate /usr/local/ispconfig/interface/ssl/ispserver.crt;