Oracle just released a Java update (8u141) and people are being automatically prompted to install this.
Unfortunately, this update contained a bug which prevented the Web Start OT from working. The release notes of the Java update are given here - scroll down to the 'Known Issues':
http://www.oracle.com/technetwork/java/javase/8u141-relnotes-3720385.html
"After upgrading to the JDK July CPU release (8u141/7u151/6u161), when executing Java Webstart applications, customers may encounter an exception like
“java.lang.SecurityException: digest missing for …” that prevents the application from loading."
There has been a patch released for the OT Cycle-5 release such that there is no longer a problem. Any user who has installed the latest update of Java 8 will be able to run the Web Start version of the OT without encountering any problems.
The problem will though still occur for anyone who tries to run the Cycle-4 OT. This will mainly affect users who wish to submit a DDT proposal.
If you need to submit a Cycle 4 DDT proposal, please use the tarball version of the OT.