I have Bitdefender Free on my Windows machines, and that sometimes warns me that Renderosity downloads are unsafe. Not always though, and I'm able to cancel the warning and download anyway.

If you wanted to add an exception, it should be to bw-1651cf0d2f737d7adeab84d339dbabd3-freestuff.s3.amazonaws.com, not Renderosity.com. As far as I can see all the downloads come from that Amazon AWS URL.