Hi all,
I have installed Kopano on Univention server 4.3.0. Kopano (WebApp 3.4.2.1108+36.1) is in local LAN, and on another machine I have Nginx Reverse Proxy with LetsEncrypt. Everything seems work fine, only filepreview (mail attachments) in WebApp not work. It show only blank frame.

If I access WebApp by local IP (http://10.128.2.102/webapp), filepreview works OK.
If I access WebApp by domain name (https://mail.my-domain.cz) filepreview show blank frame
There is my nginx config for Kopano:

server {
    server_name mail.my-domain.cz;
    listen 443 ssl http2;
    listen [::]:443 ssl http2;
    include snippets/ssl-my-domain.cz.conf;
    include snippets/ssl-params.conf;


   rewrite ^/$ /webapp;

   location /Microsoft-Server-ActiveSync {
        proxy_pass http://10.128.2.102/Microsoft-Server-ActiveSync;
        proxy_set_header X-Forwarded-Host mail.my-domain.cz;
   }

location /webmeetings {
proxy_pass http://10.128.2.102/webmeetings;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection $connection_upgrade;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_set_header Host $http_host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Host mail.my-domain.cz;
}

location /webapp {
proxy_pass http://10.128.2.102/webapp;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header X-Forwarded-Host mail.my-domain.cz;
proxy_set_header Connection $connection_upgrade;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_set_header Host $http_host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}
}

map $http_upgrade $connection_upgrade {
default upgrade;
'' close;
}

proxy_buffering on;
proxy_ignore_client_abort off;
proxy_redirect off;
proxy_connect_timeout 90;
proxy_send_timeout 90;
proxy_read_timeout 90;
proxy_buffer_size 4k;
proxy_buffers 4 32k;
proxy_busy_buffers_size 64k;
proxy_temp_file_write_size 64k;
proxy_next_upstream error timeout invalid_header http_502 http_503 http_504;

Firefox Console show this:

Load denied by X-Frame-Options: https://mail.my-domain.cz/webapp/plugins/filepreviewer/external/ViewerJS/index.html?zoom=auto&type=png#https://mail.my-domain.cz/webapp/index.php?load=download_attachment&store=0000000038a1bb1005e5101aa1bb08002b2a56c200007a617261666136636c69656e742e646c6c0000000000c7d733ad6c4b4f37879a18fa4f0fac24010000000100000002322a483b8b4a0c882f09cc8b208c3670736575646f3a2f2f756373322d61707000&entryid=00000000c7d733ad6c4b4f37879a18fa4f0fac24010000000500000016b0ee59644346b1980b7d0de03b1df000000000&attachNum[]=2&dialog_attachments=cSgs2AYgeVpV6vB0cY7AiQGjDgi13utm&contentDispositionType=inline does not permit framing.

Is needed any extra configuration on Kopano? Or extra config on Nginx?

Thanks.