ERR_SSL_VERSION_OR_CIPHER_MISMATCH when using CNAME to redirect www to non-www on app platform

I have a simple static site on App Platform, to which I have added a custom domain (let’s call it myapp.com). The domain is hooked up to Digital Ocean’s name servers, and I can navigate to the app using https://myapp.com.

The problem is that https://www.myapp.com is not resolved. So I’ve added a CNAME record to the DNS with:

hostname = www value = @

Now, https://www.myapp.com resolves, but to a Chrome warning screen which says:

The connection for this site is not secure

www.myapp.com uses an unsupported protocol.

ERR_SSL_VERSION_OR_CIPHER_MISMATCH

Can someone explain to me why this is, if I’ve done something wrong with the DNS record and suggest how to resolve this? Thanks