Collabora behind Firewall with (different port forwording) did not work

Hello, i have NC 20.0.10 on an Debian 11 (bullseye) running. The HW is an arm64 (RockPro64).
On the NC are the ‘Collabora online’ and the ‘Collabora Online - Built-in CODE Server’ installed.

Using the ‘Collabora online’ from internal LAN (and internal dns name at home) everything works fine.

(sorry - german language)

For access from extern i use a dyndns solution and reroute the external port 8443 (and dyndns name) to the internal port 443 (and the internal dns name using a fritz box).
On this setting NC works fine, (data access, calendar, contacts, …) but ‘Collabora online’ rejected access to ‘Build-in CODE Server’:

I did not know where to look for some debug information to give more detailed information which part did not work.

I also placed a thread on the Collabora online forum but did not get helpful resonse :thinking:.
So any help is most welcome :smiley: - many thanks.