To ensure TekTelnet works correctly on Windows Server 2012 (and later), make the following modifications on the server's registry and restart the server:

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\AFD\Parameters] 
"DefaultReceiveWindow"=dword:00002000 
"DefaultSendWindow"=dword:00002000 


Taken from the following link below: 
http://smallvoid.com/article/winnt-winsock-buffer.html 

Bruce Irschick