When installing the Activiti Designer within Eclipse, it exports the generated model packages org.eclipse.bpmn2.* provided by the org.activiti.designer.model project. However, this conflicts with other BPMN2 based plugins, such as the BPMN2-Editor provided by the JBoss Team for JBPM5.
The problem here is, that in case both are installed, one takes precedence and the one from http://git.eclipse.org/gitroot/bpmn2 contains some model differences to those of the activiti version leading to errors such as "namespace" is not allowed in this element.
My suggestion here would be to use the BPMN20 model as incubation dependency rather than providing an own version. In that case, both plugins can co-exist without any problems.