Hi Hugo,
Thank you for your help.
I've build the pem file in the order you shown.  Now the problem is that although hiawatha starts fine, it's not possible to visit https pages.
When                                Result
http://myserver.pt                       fine, url changes to http://www.myserver.pt/some.php
http://www.myserver.pt             fine, url just adds /some.php
https://myserver.pt                      fine, url changes to http://www.myserver.pt/some.php     [NOTE: http, not https]
https://www.myserver.pt           Secure Connection Failed
                                                     An error occurred during a connection to www.myserver.pt.
                                                     SSL received a record that exceeded the maximum permissible length. 
                                                     (Error code: ssl_error_rx_record_too_long) 
My config of hiawatha.conf is as follows: (relevant parts)
Binding {
        Port = 80
        MaxKeepAlive = 30
        TimeForRequest = 5,30
        MaxRequestSize = 512
}
Binding {
        Port = 443
        MaxKeepAlive = 30
        TimeForRequest = 5,30
}
VirtualHost {
        Hostname = myserver.pt
        UseToolkit = redirect
        WebsiteRoot = /usr/local/www/prestashop
}
VirtualHost {
        Hostname = www.myserver.pt
        WebsiteRoot = /usr/local/www/prestashop
        StartFile = index.php
        UseFastCGI = phpfpm0
        TLScertFile = /usr/local/etc/hiawatha/myserver.pt.pem
}
UrlToolkit {
        ToolkitID = redirect
        Match ^/(.*) Redirect http://www.myserver.pt/$1
}
So, there are two problems:
1. the redirect from https://myserver to http
2. the error when acessing https://www.myserver
How to solve these issues?