critical error stack trace from Unicorn

class java.io.IOException
Server returned HTTP response code: 502 for URL: http://validator.w3.org/i18n-checker/check?uri=http%3A%2F%2Fwww.thebicyclingguitarist.net%2Findex.htm&format=xml
sun.reflect.GeneratedConstructorAccessor165.newInstance(Unknown Source)
sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
java.lang.reflect.Constructor.newInstance(Constructor.java:513)
sun.net.www.protocol.http.HttpURLConnection$6.run(HttpURLConnection.java:1491)
java.security.AccessController.doPrivileged(Native Method)
sun.net.www.protocol.http.HttpURLConnection.getChainedException(HttpURLConnection.java:1485)
sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1139)
class java.io.IOException
Server returned HTTP response code: 502 for URL: http://validator.w3.org/i18n-checker/check?uri=http%3A%2F%2Fwww.thebicyclingguitarist.net%2Findex.htm&format=xml
sun.reflect.GeneratedConstructorAccessor165.newInstance(Unknown Source)
sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
java.lang.reflect.Constructor.newInstance(Constructor.java:513)
sun.net.www.protocol.http.HttpURLConnection$6.run(HttpURLConnection.java:1491)
java.security.AccessController.doPrivileged(Native Method)
sun.net.www.protocol.http.HttpURLConnection.getChainedException(HttpURLConnection.java:1485)
sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1139)
org.w3c.unicorn.request.URIRequest.doRequest(URIRequest.java:141)
org.w3c.unicorn.RequestThread.run(RequestThread.java:66)