OK, fixed it myself Thanks to this topic: JSON change-password request to /ssl/user-manage-passwords failed - #12 by Axlm
I found this post from @ggalmazor :
* Stop Tomcat * Create a backup of the whole directory where Aggregate is deployed * Unzip WEB-INF/lib/ODKAggregate-settings.jar into WEB-INF/classes * Delete WEB-INF/lib/ODKAggregate-settings.jar << this is important * Edit WEB-INF/classes/security.properties and set security.server.hostname with the IP address or hostname that the users will be using to access Aggregate * Start Tomcat
There i found jdbc.properties in which the postgresql url is configured.
jdbc.url=jdbc:postgresql://sqlserver.mydomain.com/odk_prod?autoDeserialize=true
According to the documentation @ https://jdbc.postgresql.org/documentation/head/connect.html adding "?ssl=true" to the url should do the trick.
well, it did and ODKAggregate now connects to a postgresql database that enforces ssl connections.