Send Tasks && camunda-bpm-mail

@HEss refer this post for similar issue was discussed and resolved.

Also, make sure that you’ve the below plugin in pom.xml:

<dependency>
  <groupId>org.camunda.bpm</groupId>
  <artifactId>camunda-engine-plugin-connect</artifactId>
</dependency>

This plugin contains the piece of code to parse the connector extension of the bpmn file.