Hello again !
After long experiments I deactivated my firewall completely (although there were defined rules for port 135 and OpCEnum in both directions).
Result: Yes now I can subscribe to the remote OPC DA server items, without problem . The response is now definitly shorter then read item: just 1-2 seconds.
For validating, I turned off the firewall again, and the subscribing failed again.

Now I try to find out what specific firewall rule I have to define, because deactivating firewall comletely, will not be accepted in our company. It is strange that with the activated firewall the OPC DA test client could subscribe to items without any problem. I couldn't see any appl. specific rule for that. I don't know if the test client can define rules programmatically over registers.

Thank you again for your great support.

No unfortunately not. I have read again with appr.40 seconds delay. For repeating: The same machine, the same tag I could subscribe with the OPC test client-tool immedeiatly. I think the subscribing will make the difference, of course if I can make it. From some documents I have figured out what to do on client side (my server where the app is running) DCOM settings for callbacks. I have done some settings in dconcnfg like giving anonymous logon on and everyone some rights for romote acces, but nothing changed. For subscribing still I get no value, and also no Error.

Yes have just tried it out.

With the sample code you send, Icould also read item from the remote machine. There was no problem.

Hello
I have made following tests, acoording your suggestion: (All running on the same machine, .Net Framework 4.7.2, OPC Labs V5.71)

1)*********************************
First I have tried to read item with following simplified code from the remote CNC machine: (Result: item can be read after 40 seconds.
public void read_item()
{
ComManagement.Instance.Configuration.InstantiationParameters.OverrideDefaultSecurity = false;
ComManagement.Instance.Configuration.SecurityParameters.UseCustomSecurity = true;
ComManagement.Instance.Configuration.SecurityParameters.TurnOffCallSecurity = true;
Console.WriteLine(EasyDAClient.SharedInstance.ReadItemValue(
"10.92.120.174", "OPC.IwSCP.1", "!BSTR,IndraMotion_MTX_P70,System.HardwareDescription").ToString());
Console.ReadLine();
}

2)*********************************
Then I have tried to subscribe to the same item (again remote CNC machine) Result: There is no error but I don't get any value, after waiting for few minutes.

public void subscribe_items_CNC()
{
ComManagement.Instance.Configuration.InstantiationParameters.OverrideDefaultSecurity = false;
ComManagement.Instance.Configuration.SecurityParameters.UseCustomSecurity = true;
ComManagement.Instance.Configuration.SecurityParameters.TurnOffCallSecurity = true;
EasyDAClient.SharedInstance.SubscribeItem(
"10.92.120.174", "OPC.IwSCP.1", "!BSTR,IndraMotion_MTX_P70,System.HardwareDescription", 1000,
(_, args) => Console.WriteLine(args.Vtq.Value));
Console.ReadLine();
}

3)*********************************
On the same PC where my code is running, I have installed a OPC DA server for simulation (Matrikon) and tried to subscribe to a test item.
This worked absolutely fine. It cathces every change of the item immediatly. But one remark!!!;
When I used the security parameters like above I got the situation like 2) (no error, but also no data, just waiting)
When I used no security patameters or just the first, or the first two of the parameters, I could subscribe,
Here TurnOffCallSecurity = true was preventing to subscribe.

public void subscribe_items_simulation()
{
EasyDAClient.SharedInstance.SubscribeItem(
"localhost", "Matrikon.OPC.Simulation.1", "group_1.item_1", 1000,
(_, args) => Console.WriteLine(args.Vtq.Value));
Console.ReadLine();
}

**************************************************
Result:
Is it possible that "read item" and "subscribe item" do not work with same security parameters for reading from my remote CNC machine?
Or did the TurnOffCallSecurity = true caused a problem, just because I was subscribing to an item of the local machine (Simulation OPC DA server)?

Hello

I have tried instead of reading multiple item, as you suggested subscribing multiple items with following code: I have taken the same items that I could read delayed with "read multiple items". Although I am waiting for 60 seconds I get no value (The event is not triggered) and the console exits without any value. The items that I try to read are non changing items (Version infos). Could that be the reason? Or should I be able to read them at least for the first time, although their values are not changing?

public static void subscribe_items()
{
// Instantiate the client object.
using (var client = new EasyDAClient())
{
client.ItemChanged += client_Main1_ItemChanged;

Console.WriteLine("Subscribing item changes...");
client.SubscribeMultipleItems(
new[] {
new DAItemGroupArguments("10.92.120.174", "OPC.IwSCP.1", "!BSTR,IndraMotion_MTX_P70,System.HardwareDescription", 1000, null),
new DAItemGroupArguments("10.92.120.174", "OPC.IwSCP.1", "!BSTR,IndraMotion_MTX_P70,Plc.FirmwareVersion", 1000, null),
new DAItemGroupArguments("10.92.120.174", "OPC.IwSCP.1", "!BSTR,IndraMotion_MTX_P70,Firmware.DeviceFirmware{0,0}", 1000, null),
});

Console.WriteLine("Processing item changed events for 1 minute...");
Thread.Sleep(60 * 1000);

Console.WriteLine("Unsubscribing item changes...");
}

Console.WriteLine("Finished.");
}

// Item changed event handler
static void client_Main1_ItemChanged(object sender, EasyDAItemChangedEventArgs e)
{
if (e.Succeeded)
Console.WriteLine($"{e.Arguments.ItemDescriptor.ItemId}: {e.Vtq}");
else
Console.WriteLine($"{e.Arguments.ItemDescriptor.ItemId} *** Failure: {e.ErrorMessageBrief}");
}