It appears that the server running your Mendix application does not trust the certificate of the web service. This could happen when the certificate is self-signed instead of issued by a trusted certificate authority.
You can add the certificate to your project in the Certificates tab of the Project Settings (version 2.5.2 and above). See the Project Settings documentation for more information.
In java, you need to register certificates you want to trust, see the official documentation
and
https://forum.mendix.com/questions/1565/Mendix-configuration--PostgreSQL-database-SSL