If you are using a Java version prior to 7u75 or 8u31, you may get an error saying:
REST action POST: https://pki.rest/Api/xxxxx unreachable
This happens because the root CA certificate of our SSL certificate chain was only added to the Java trusted root certificates on the aforementioned versions. To fix this, update your Java to a current version.
If you don't wish to update your Java, you may alter the file
Java/sample-spring-mvc/src/main/java/sample/util/Util.java and switch the REST PKI address to "http://pki.rest/" (with "http" instead of "https"). However, this fix should only be used while on development, since your API access token, as well as the documents you sign, will be sent to REST PKI unencrypted.