SSL certificate on Google App Engine not working properly

From Adding a custom domain for your application:

• A naked domain, such as example.com, maps to http://example.com.

So it maps to the non-secured (i.e. http, not https) URL of the domain. This is because a custom domain can be equally well used for sites which don't use SSL.

If you want it to always get the secured URL just configure its handler with secure: always in the app.yaml file. From Handlers element:

secure

Optional. Any URL handler can use the secure setting, including script handlers and static file handlers. The secure element has the following possible values:

...

• always

  Requests for a URL that match this handler that do not use HTTPS are automatically redirected to the HTTPS URL with the same path. Query parameters are preserved for the redirect.

Example

handlers: - url: /youraccount/.* script: accounts.app login: required secure: always 

Update:

Your naked domain redirection works OK now. The page loads fine for me in both desktop FF and Chrome.

It does show an error on my old android phone: ERR_CERT_AUTHORITY_INVALID. I had a similar problem a while ago, my certificate was missing intermediate authorities, I just got a new certificate and verified it to be complete.

I just confirmed it for your site using digicert:

SSL Certificate is not trusted

The certificate is not signed by a trusted authority (checking against Mozilla's root store). If you bought the certificate from a trusted authority, you probably just need to install one or more Intermediate certificates. Contact your certificate provider for assistance doing this for your server platform.