I'm using the SharePoint web proxy in an SharePoint App to call an external service (which I control). I set the service up to use SSL and it seems SharePoint has a problem with my certificate (or something) since it fails with the above error when calling the service with https. If I use http instead of https the call works fine. The app.manifest is correct. If I call the same service from the browser using https it works fine.
Also if I put the service on free Azure web sites which provides SSL automatically and have the SP WebProxy call that it also works OK. So SP must be happy with the Azure SSL certificate
It seems that it is something specific to my SSL certificate that SharePoint doesn't like.
Since it is Office 365 I don't control the server so I can't install certificates.