Blender 2.37a(CVS) & Yafray: problems with artefacts

Hi,

I got strange artefacts when rendering the following setup:

An area light with a white plane in the back directed to
a “plane” (explanation follows) made from glass isn’t
transparent.
“Plane”: Add plane, enter edit mode, select all vertice, extrude region and lift the extrusion a little to get a gplass plate.

“Glass”: Material made from the default setting for “clear glass”.

Result: The glass plate is “filled” with a black “whirl”-like artefact.
I increased all “depths” I could found like “raydepth” and so on and played around with higher AA-settings: No success.

Since I have no webspace I am currently not able to provide an example…

If someone likes to provide me a little piece of mem (the blend-file isn’t that big…) I would be happy to post that file.

Thank you very much for any help or hint in advance !

Keep rendering!
mcc