JDI Thread Evaluations has encountered a problem

The Java Debug Interface (JDI) is part of the Java Platform Debugger Architecture. One apparent way to trigger this exception occurs when an object's toString() method carelessly returns null. You might want to try a different debugger, and it wouldn't hurt to validate your XML. As noted in comments, the problem may appear intermittently, suggesting a thread synchronization problem.

Keep an eye on your "Watch" expressions - if you don't need them, remove them all. Sometimes certain watch expressions have caused this error message for me.

This might also explain why it happens to intermittently for people (sometimes they have watch assignments that cause the error, however restarting or removing the right watch can solve the issue without them knowing it.)