clickonce install fail - URLDownloadToCacheFile HRESULT '-2146697191'

trying to run the setup.exe to install

from the install.log file, here is the error:

Launching Application.
URLDownloadToCacheFile failed with HRESULT '-2146697191'
Error: An error occurred trying to download 'https://galantha.net/appPublishGalanthasEncryptedNotes/GalsPassHolder.application'.

-2146697191 = 0x800C0019

Which leads to: https://docs.microsoft.com/en-us/previous-versions/windows/internet-explorer/ie-developer/platform-apis/ms775145(v=vs.85)?redirectedfrom=MSDN

INET_E_INVALID_CERTIFICATE 0x800C0019   
The SSL certificate is invalid.

And then I hit a wall. I cannot figure out why it feels the certificate is invalid?

the deployment is hosted here: https://galantha.net/appPublishGalanthasEncryptedNotes/publish.htm and the certificate checks out fine: https://www.sslshopper.com/ssl-checker.html#hostname=https://galantha.net/appPublishGalanthasEncryptedNotes/publish.htm

I can also see that: https://galantha.net/appPublishGalanthasEncryptedNotes/GalsPassHolder.application will download fine, provided the user ignores the warning about .application. Nothing about an ssl error.

Any help would be appreciated.