Custom Menu not shown in the Widget, Sidebar area

This is bizarre; I have created a custom menu, also a sidebar and finally assigned to some pages this sidebar, so as to have pages with side navigation. In the frontend this is working fine, but when I go to: Appearance > Widgets > MySidebar (named: ΒασικέςΘεσεις) the assigned menu is not shown – disappeared! See also the attached.

How can we fix this?

For your convenience I am sending you the login info with a pm.

Thank you so much!