Problem: after a vmware virtual server (windows 2012) was using 10gbit instead of 1gbit it was not able anymore to build up valid tcp session to some specific hosts

Solution: after using wireshark we found out, that the difference was the tcp ECN bit, which was set when using 10gbit

>> disable ecn on the virtual windows machine:
netsh int tcp set global ecncapability=Disabled

>> check windows settings:
netsh int tcp show global

see also:

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