I ran into the same issue and investigated it.
Go uses a Windows API function called TransmitFile to transmit file data over connected sockets. Workstation and client versions of Windows limit the number of concurrent TransmitFile operations allowed on the system to a maximum of two. This is what causes the issue.
I reported this and submitted a change that makes Go avoid TransmitFile in such cases. The change has been merged and should be included in the next release of Go.
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