Create tiles definitions file (in Web Projects View, right click node Tiles under project and select New->Tiles...).
Open editor and make sure that Tree tab looks ok.
Add definition named 'd1'.
Switch to Source tab.
Modify name 'd1' to 'd2'.
Switch back to Tree tab.
Now it shows error in xml.

Explanation - There is incorrect reference to tiles-config_1_1.dtd file in plugin.xml. Hence in runtime, the reference is resolved so that model is notified that this kind of file can be validated with dtd, but validation files because dtd file is not found.

By the way, it is better to move registration of the dtd to from struts plugin to tiles plugin.