Patch download sporadically fails with (407) Proxy Authentication Required

I just solved an issue on SMP 7.1 where some Firefox patches didn't download some times.

There were errors like:

08.11.2012 10:51:01 The remote server returned an error: (407) Proxy Authentication Required. PMC:File.DownloadToStreamWithRetries AtrsHost 150

and

"15.11.2012 13:20:22","The server committed a protocol violation. Section=ResponseStatusLine","PMC:File.DownloadToStreamWithRetries","AtrsHost","345"
The server committed a protocol violation

even when patch downloads finished successfully. The proxy settings were correct.

Very strange.

However I solved it by activating CustomWebProxyKeepAlive.

I found that solution in this article for Notification Solution 6.0 and Patch Management 6.1

Unable to download PMImport/Software updates when using a proxy server