.NET HttpClient issues
shorten up the time connections "wait" before Windows really, really closes them. To do this, add the following HEX/DWORD entry to the local registry of your client system. By default, the implicit value is "120", which means a time out of 2 minutes. You can pick a value of TcpTimedWaitDelay down to "1", but a higher value is recommended.
HKEY_LM\System\CurrentControlSet\Services\Tcpip\Parameters\TcpTimedWaitDelay = 30 (decimal)
I'm not sure that this affects CLOSE_WAIT, because the official Microsoft documentation
says that the TcpTimedWaitDelay value determines the length of time that a connection stays in the TIME_WAIT state when being closed! But let's give it a try!
Could you please send us diagnostic information from that machine? Please click onto the tray icon, select Diagnostics, fill the form and past a link to that conversation.