The GEF OrderedLayoutEditPolicy class has changed in version 3.7. This introduces isLayoutHorizontal() which replaces the deprecated isHorizontal() method - this assumes that the layout manager is an OrderedLayout (which is not the case) causing a class cast exception. I have added an override for isLayoutHorizontal() to BPELOrderedLayoutEditPolicy, which calls the deprecated method (for backward compatibility).
This is related to https://bugs.eclipse.org/bugs/show_bug.cgi?id=88884