I tried hovering my mouse over the red "X", but no error description appears.
I tried opening the "Points to Line" tool in a new (blank) map document (without any data added). The same red "X" appears beside the "Input Features" text of the tool.
Furthermore, I discovered the same red "X" appears in the "Add Geometry Attributes" tool.
Both these tools, "Add Geometry Attributes" and "Points to Line" bears a "Scroll" icon in the ArcToolbox, all the other tools has a "Hammer" icon
All the other tools are fine - with a green dot beside the "Input Features" text.