Okay, I think I finally found a workaround:
Vista and 7 feature adaptive scaling of the TCP receive window. Some routers or other network devices don't work well with this feature. To disable it open a command shell with administrative rights and enter
netsh int tcp set global autotuninglevel=disabled
This can reduce the performance of the TCP/IP-stack but should improve compatibility.
See also Vista Auto-Tuning | kombitz.com