OnlyOffice is not accessible from LAN

Hello,

I have installed Nextcloud 18 on Debian 10 with a LetsEncypt certificate.
I can access it from the internet (mydomain.ddns.com) and from my lan (192.168.1.xx).

I have installed OnlyOffice but it does only works when I am accessing Nextcloud with mydomain.ddns.com. It does not work on the LAN.

How can I fix it?

Thanks!

Regards.