Smartthings webhook fail on synology

Hi all!
I’m looking for someone who has already encountered an incoming webhook issue on HA for Smartthings integration under Syno.

I have a DDNS configuration of the syno with a LetsEncrypt certificate and a functional reverse proxy for the outside for many services and I have never had any issues.

And now I want to use the Smartthings integration (because I acquired Ledvance equipment which is better implemented in Smartthings than under TuyaLocal) but I am running into the issue of Webhook validation which does not pass.

The HA log sees an incoming webhook connection but considers it as a GET which seems to indicate after some research, that there could possibly be an issue with the SSL certificate???

I have a little trouble understanding knowing that my certificate on the syno is not self signed and when I test it on a site, it indicates TLS 1.3 + TLS 1.2 compatibility.
So I’m a little stuck. Does anyone have any experience in this by chance?
Thanks in advance and sorry for the big block of text.