When workflows reach a user task step, none of the notification emails to action the task are received by assigned users.
This occurs only on initial email notifications for User Tasks. Reminder emails (escalations) and Email Events are not affected. Assigned users will not receive emails, however the workflow appears to proceed without error. No errors are logged in Management, however Kibana logs will return the following error when the user task is hit after a workflow instance is started:
"61007 Permanent failure sending message [Process Instance - Title]: System.FormatException: The specified string is not in the form required for an e-mail address."
A service restart will resolve this issue. Should you experience this issue, you can log a ticket with K2 Support and they can request a service restart on your behalf. Additionally, a fix will be included for this issue in a future K2 Cloud FixPack (Update 4 FixPack 11+).