Hi, I've eliminated the glass from the window and the dark flies went away (see new scene version that is still cooking).
From what I've understood, the problem is present only if the Sun
directly crosses the specular material (glass).
P.S. Here is the curtain material (very simple, is Indigo that it's a great engine

)
<!-- Material : "Tenda bianca" -->
<material>
<name>Material_56_Diffuse</name>
<diffuse>
<albedo>
<constant>
<rgb>
<rgb>0.800000 0.800000 0.800000</rgb>
<gamma>2.2</gamma>
</rgb>
</constant>
</albedo>
</diffuse>
</material>
<material>
<name>Material_56_DiffuseTransmitter</name>
<diffuse_transmitter>
<albedo>
<constant>
<rgb>
<rgb>0.800000 0.800000 0.800000</rgb>
<gamma>2.2</gamma>
</rgb>
</constant>
</albedo>
</diffuse_transmitter>
</material>
<material>
<name>Material_56</name>
<blend>
<a_name>Material_56_Diffuse</a_name>
<b_name>Material_56_DiffuseTransmitter</b_name>
<blend>
<constant>0.300000</constant>
</blend>
</blend>
</material>