Localhost tcp traffic stalled

After installing a 4G/LTE USB stick on a Windows 7 desktop the TCP communication on localhost has effectively stalled. Interestingly, the remote traffic was not affected. Local services can be reached by remote clients and remote services can be accessed from the desktop, only the localhost (127.0.0.1) is affected. Still ping (ICMP) works on localhost as before. Capturing the traffic reveals that all localhost services communicate through TCP become utterly slow. The TCP session starts by a zero window packet in both direction right after the SYN-SYN/ACK handshake. After that, only 1 byte payload arrives in 20-60 seconds interval. Most clients give up after 30 seconds and return error. The problem persist after removing every installed software and turning off the Windows Defender service. It doesn’t matter which port the service is listening on.

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