Hi Yann,
Did you get any solution to this problem. Am facing the same issue when trying to add a "isolated" network to vApp using below C# SDK code.
VAppNetworkConfigurationType vAppNetworkConfigurationType = new VAppNetworkConfigurationType();
vAppNetworkConfigurationType.networkName = “myNetwork”;
vAppNetworkConfigurationType.Configuration = new NetworkConfigurationType();
vAppNetworkConfigurationType.Configuration.FenceMode = “isolated”;
vAppNetworkConfigurationType.Configuration.IpScope = new IpScopeType();
vAppNetworkConfigurationType.Configuration.IpScope.Gateway = “192.168.2.1″;
vAppNetworkConfigurationType.Configuration.IpScope.Netmask = “255.255.255.0″;
IpRangeType ipRange = new IpRangeType();
ipRange.StartAddress = “192.168.2.100″;
ipRange.EndAddress = “192.168.2.199″;
vAppNetworkConfigurationType.Configuration.IpScope.IpRanges = new IpRangesType();
vAppNetworkConfigurationType.Configuration.IpScope.IpRanges.IpRange = new IpRangeType[]{ipRange};
NetworkConfigSectionType networkConfigSectionType = new NetworkConfigSectionType();
networkConfigSectionType.NetworkConfig = new VAppNetworkConfigurationType[] {vAppNetworkConfigurationType};
networkConfigSectionType.Info = new Msg_Type();
Task task_net = vApp.UpdateSection(networkConfigSectionType);
task_net.WaitForTask(timeout); <<< Exception is thrown in this line