Hello!
My username (login) is "sa" (Active Directory connector). Earlier, using IssueService, I created issues and there were no problems. After renaming the username to "id", I started getting an error:
"[assignee:User 'sa' does not exist.]"
[VALIDATION_FAILED]
I am not using the {{params.setAssigneeId()}} method.
Check:
import com.atlassian.jira.component.ComponentAccessor
def user = ComponentAccessor.userManager.getUserByName( "id" );
// result = "id(sa)"
It looks like I exist with a new name "id" and with an old key "sa".