What has changed in the Template system in 10.3?
There seems to be an incompatible change to NotebookExpression. While I don't quite know how to fix your example, I went ahead and fixed my trouble, i.e. just some input cells which have to be evaluated and hidden. This can be achieved with the following button:
Button["TemplateExpression Exorcist",
Module[{cell},
(* move from inside the cell to the visible cell bracket *)
Do[SelectionMove[SelectedNotebook[], All, Cell], {10}];
cell = (NotebookRead[SelectedNotebook[]] /.
Cell[BoxData[
FormBox[TemplateBox[boxes_,
"NotebookTemplateExpression"], _]]] :> boxes ) //.
BoxData[{b_, "General", BoxData}] :> BoxData[b];
NotebookWrite[SelectedNotebook[], cell]]] // CreatePalette
So a cell like this one (whichi works in a template in 10.2)
Cell[BoxData[Cell[BoxData[
FormBox[
TemplateBox[{RowBox[{
RowBox[{"myreport", "[",
RowBox[{
Cell[
BoxData[
FormBox[
TemplateBox[{"\"var\"", "\"t0\"", "Named", BoxData}, "NotebookTemplateSlot"],
TextForm]]], ",", "\"novar\""}], "]"}], " "}],"General",BoxData},
"NotebookTemplateExpression"], TextForm]]]], "Input",
CellFrameLabels->{{
Cell[
BoxData[
TemplateBox[{"EvaluateDeleteInput"}, "NotebookTemplateCellBehavior"]]], None}, {None, None}}]
is change to a working one (in 10.3)
Cell[BoxData[
RowBox[{
RowBox[{"myreport", "[",
RowBox[{Cell[BoxData[
FormBox[
TemplateBox[{"\"var\"","\"t0\"","Named",BoxData},
"NotebookTemplateSlot"], TextForm]]], ",", "\"\<novar\>\""}], "]"}], " "}]], "Input",
CellFrameLabels->{{
Cell[
BoxData[
TemplateBox[{"EvaluateDeleteInput"}, "NotebookTemplateCellBehavior"]]], None}, {None, None}}]
This bug was fixed with V11.0.0 release.