Reported for: Toolset Layouts 2.4.3
Resolved in: 2.5.1
When you try to set a parent layout for the layout you are currently editing, the GUI is broken and shows a string like
use data-tooltip-content="content text" attribute to create a content.
Because of this, you are not able to connect the layout to a certain other (parent) layout.
As a workaround:
The parent layout will be assigned and work correctly. Also, the related GUI will not be broken anymore.