I tested application from other PCs - the results are strange:
On 4 PCs with silverlight there are no errors during file uploading.
But on 2 others - the same error occurred quite often.
IE6, Silverlight 2.0.40115.0
IE8, Silverlight 3.0.40818.0
Moreover, if I open page with Uploader via window.showModalDialog there are other errors on that 2 PCs:
IE6, Silverlight 2.0.40115.0 - Quite often occurres "Flash : The upload task is interrupted! io Error #2038"
IE8, Silverlight 3.0.40818.0 - IE crushes - "Unhandled exception at 0x3de561a2 in iexplore.exe: 0xC0000005: Access violation reading location 0x00000000." :)
I didn't tested application via http on https-environment. If I use IIS on my computer everything works good.
Probably it's not your bug.
Regards,
Dan