Why can't Java talk to https server?

HI. I have a Java application that communicates with an https server but recently stopped working. The basis of the code is that it tries to open a URL to https://.... and then spits out this handshaking error:

javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

I understand this looks like a problem with keystores, however the following makes me believe this isn't the case...
1. This same code works on another linux server and a windows machine where no keystore has ever been set up
2. When I replace  the URL in my code with another https URL it works fine without having to setup any keystore
3. I can telnet to and open the URL on the server - it's only Java which fails to communicate with it

It is possible someone in the past has set a keystore on this machine

This isn't the exact code from my application but I have reduced it to this basic statement which gives the same exception:
 
URL url = new URL("https://www.server.co.uk");
System.out.println(url.getContent());
 
I have replaced the actual URL with this dummy one

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