Implementation Task issue in ServiceDesk

Hello everybody

I need support with Servicedesk 7.5 MP1 in Change Management.

When I create a Change Request, I can do many things but when I try delegate a implementation task, I get several issues, for example:

1.- I can use "Delegate Implementation Task" Menu, but that task is created without assigned technician

2.- In the Menu "Manage Implementation Task" appears this message error: "The implementer you entered was not found. Please select or enter the primary email address of a user", but the user exist and the primary mail is correct.

I attached some images for support.

Somebody has this issue?

thank for all