ReverseProxy for external OnlyOffice-Server

NethServer Version: 7.9
Module: ReverseProxy

Hi, I want to use an external vm as OnlyOffice-Documentserver for Nextcloud inside Nethserver.
I get connected with the server. The connection from the internet works also. I can see the “Welcome”-Page.
Inside Nextcloud the configuration of the OpenOffice-App is also working. The correct version of the documentserver is shown.
When I open a new document, I can see for a moment the OpenOffice Symbols and then I get a white screen in the webbrowser.
What is wrong?