I just updated to 1.399 (from 1.398) and get the following

SEVERE: Failed to initialize Hudson
java.lang.UnsupportedClassVersionError: Bad version number in .class file
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at winstone.classLoader.WebappClassLoader.loadClass(WebappClassLoader.java:68)
at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
at hudson.DNSMultiCast.<init>(DNSMultiCast.java:26)
at hudson.model.Hudson.<init>(Hudson.java:691)
at hudson.model.Hudson.<init>(Hudson.java:605)
at hudson.WebAppMain$2.run(WebAppMain.java:221)

Suggests it was built with a jdk > 1.5 perhaps?