There is something wrong with the Mass Piece Loader with Vassal 3.4.12 and Vassal 3.5.0-beta2. This issue was NOT present in Vassal 3.4.11.
I started a brand new module and then
1) added a deck to the main map,
2) selected "Add Multiple Cards"
3) selected "Edit Piece Template"
4) added a "Layer" trait
with Vassal 3.4.11 all is well and I get the "modified layer" trait,
with Vassal 3.4.12 I get a "Uncaught Exception" error message,
with Vassal 3.5.0-beta2 the normal "Layer" trait appears, instead of the "modified layer" trait.