I think I’ve figured out what is going on here.
I have a laptop that worked fine when it came to posting models to Sketchfab. I then tried from my desktop and no joy. OpenSSL was already installed on the Desktop by another app. I tried downloading the latest version of OpenSSL thinking that it would be the most up to date and therefore most likely to have any bugs fixed.
Both machines are 64bit and downloading the 32 bit as someone suggested above didn’t help. Photoscan is most definitely a 64 bit app!
Turns out when I downloaded and installed the specific version “Win64 OpenSSL v1.0.2m Light”
did it work. No need to mess with the registry.
I believe that the issue is the Photoscan has been compiled to work with the specific version of OpenSSL and many machines have a more recent version of OpenSSL. Hopefully Agisoft will see this post and fix it going forward to use the a no specific version of OpenSSL.