In case of task escalation (if email notification is configured on the task) engine tries to send email, which ends up with an error message in server log:
"Cannot proceed with notifications as not all requirements are meet - mail session or userinfo is not available."

Looking at the EmailSessionProducer, email session has to be provided via JNDI or - when it hasn't been found there - engine searches for email.properties. But there is no information about what went wrong in the server log. The only message ends with "mail session or userinfo is not available.", which does not help much - is missing userinfo or mail session?