Hi @Stephane and others,
Unfortunately, not a lot comes to mind immediately with this specific scenario. One thing I’d recommend is making sure that your server can handle TLS 1.1+ (1.1 or 1.2 as of now) connections, along with 2048 bit certificates or greater (which was mentioned earlier).
I’m looking into our exact verification process to determine which CA bundle it uses and anything else it might do in that process, and will let you all know. Of note is that we have verified on our end that certificates from Let’s Encrypt do work (even though there was some debate on whether this worked over in https://forum.asana.com/t/x-hook-secret-only-32-chars/).
Otherwise, we can look into specific webhooks on a case by case basis via a ticket to support, however it does take some time to find the specific webhook to send over exactly what went wrong on our end.