Remote Access KO on one site

Server Version#: Last on Last DSM version
Player Version#: Samsung Tizen & App iOS / iPadOS

Hello,
I have two Plex servers hosted on two Synology NAS devices running DSM 7.2 (each at a different location). Both NAS devices have fiber optic internet, and my ISP prioritizes IPv6 access.
I’m currently cleaning up my open ports to route all traffic through port 443 using reverse proxy rules.
Port 443 is open in both the IPv4 and IPv6 firewalls of my routers.

I created a proxy entry that forwards https://plex.DDNS:443 to https://localhost.32400.
This works on one of my NAS devices, but not the other. I don’t understand why…

Any ideas ?

Thanks…

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