Re: problems with RijndaelManagedTransform.DecryptData
I believe this error would occur if the application instance that receives a Flash-based upload is a different instance from the one that rendered the page. That could happen if either your application is occasionally restarting or you are using a web garden/farm. In either case, to fix the problem specify the same random 32-hex-digit decryptionKey attribute in the <neatUpload> section of each server's Web.config. That key allows NeatUpload to securely communicate the state of uploads across all application instances.