Searching the error message in the source code shows 3 occurances, it appears that the "Copy Modifiers" operator is only available for some freestyle settings.
blender-2.69\source\blender\editors\render\render_shading.c
BKE_report(op->reports, RPT_ERROR, "The object the data pointer refers to is not a valid modifier");
BKE_report(op->reports, RPT_ERROR, "The object the data pointer refers to is not a valid modifier");
BKE_report(op->reports, RPT_ERROR, "The object the data pointer refers to is not a valid modifier");
As described here Add the Same Modifier to Multiple Objects at Once?Add the Same Modifier to Multiple Objects at Once? you could link the modifiers using Ctrl-L (Make Links) and select Modifiers.
Python:
bpy.ops.object.make_links_data(type='MODIFIERS')
