is the subprocess in the same project and repository? If not it won't be able to find it.
I just realized that clicking the plus sign to drill into a child process works if you are in the Project Authoring mode. It seems that it only fails when you are in the Process Instances mode. Maybe I can find the difference between the two, to see why it fails in the Process Instances screens.
I also upgraded to 6.1.0.Final and I had the same problem.
I have created a Jira ticket for this: https://issues.jboss.org/browse/JBPM-4446