HI Andrew,
unfortunately I think the Errormessage just uses the word "DescribeData", but my script uses the describe object in the right way. As I wrote in my first threat, when the script runs as stand-alone everything is fine. I am using the geoprocessor 9.3 an the code looks like this:
describe = gp.Describe(raster)
if describe.SpatialReference.name == "Unknown":
gp.DefineProjection_management(raster, inputcoord)
Actually I don't know why but it seems to me that the script works now also from the ArcToolbox. Maybe it was just another example of "Reboot your system and everything is fine again".
Thanks for your reply.
Martin