Starting task fails with"Unexpected error during processing: User null was not found in callback"

Issue

1. I start a process with adminuser. In the process there's a task assigned to "user3" and "user6".
2. I log in with user3 (or user6, it's the same), I see the task in the inbox, and I claim it correctly. but when I try to start it I get the error:

Unexpected error occurred: Unexpected HTTP response code when requesting URI 'http://kie-server-host:80/services/rest/server/containers/test_1.0.0-SNAPSHOT/tasks/11/states/started'! Error code: 500, message: "Unexpected error during processing: User null was not found in callback org.jbpm.services.task.identity.LDAPUserGroupCallbackImpl@235912e1"