IIS returns 404 when attempting to upload a blob

I’ve come across following problem. User attempts to upload blob through UploadBlob.aspx handler, but IIS returns 404 response. Handler is properly set up. Strangely enough DownloadBlob.aspx works without problem. So far I haven’t been able to find the cause, but I’ve found a workaround together with @urbanekv. When we remove following segment from web.config, handler starts to work.

<location path="UploadBlob.aspx">
            <allow users="*" />

Is there another way to solve this issue?

Maybe the upload behavior changed with a newer version of .NET/Flash? Before upload from Flash was always non-authenticated – the cookie was not sent with the upload. Maybe it works differently now and so the configuration is not good anymore. Can you test it?