You're right. I got the same error when working completely on 5.9.1.
Not clear what I am missing. This is the element:
<serviceTask id="servicetask1" name="Jenkins Task" activiti:class="org.example.jenkow.JenkinsTaskDelegate">
<extensionElements>
<activiti:field name="jobName">
<activiti:string>launchy</activiti:string>
</activiti:field>
</extensionElements>
</serviceTask>
FATAL: One of the attributes 'class', 'delegateExpression', 'type', 'operation', or 'expression' is mandatory on serviceTask. | /scratch/jenkow-plugin/jenkow-plugin/target/test-classes/diagrams/ExecLogger.bpmn20.xml | line 11 | column 131
-Max