Just to conclude this ordeal - in case someone else reads this thread.
I did manage to solve this in the end - with good assistance from @cgtobi. Here’s what happened;
Prerequisite for the problem: I have everything running on various QNAP NAS’s
As it turns out, Let’s Encrypt Certificates generated using the built-in feature of the QNAP QTS works just fine for everything - except - WebHooks. I haven’t been able to figure out why. It just won’t work.
So, what I did was to install Jamie’s Nginx based Proxy Manager in a container using QNAP CS (My full How-To Guide can be found here: https://forum.qnap.com/viewtopic.php?f=354&t=155970). Then, from the Proxy Manager GUI it’s fairly easy to generate a Let’s Encrypt certificate. It may seem like an overly long detour, but it works. The certificate generated by the Proxy Manager can then be copied and used anywhere else.
The only obvious difference I noticed between the QNAP and the Nginx generated certificates, is that QNAP bundles key+certificate into a single file (stunnel.pem), while Nginx keeps them in separate files(fullchain.pem, keyfil.pem). I tried to manually split the QNAP generated stunnel.pem file into two separate files - after all, it’s just an ASCII file. But my attempt failed. I still do not know why, but at least now I have a system that works as intended.
If you have additional information that can enlighten (educate) me, please feel free.