There were some tiny lines in the DAE/Collada model:
Code:
<lines count="2">
<input semantic="VERTEX" source="#Plane-mesh-vertices" offset="0"/>
<p>103 102 107 106</p>
</lines>
And these caused problems in AC3D because they didn't specify a material. Using Object->Optimise-Surfaces removes them from the model once it's loaded into AC3D. I've added a fix into the AC3D collada imported code to make materials on lines optional, but use Optimise surfaces until the next AC3D is available.