Thank you so much for your great support and also patience!!! Perhaps we couldn't fix the problem %100 but now I have a full understanding why we have the problem,
and also the all possible alternatives how we can get arround it. Yes I will evaluate your 3 suggestions and try to find the best solution.

With this topic I have learnt a lot of details reg. OPC DA & DCOM settings and the security mechanism (I hope this will be also for the other followers a good reference).
Additionally: You have really a great documentation that discribes everything in detail. I have made once more the experience that the after-sale support is as important as the product itself.

Thank you so much again

Hello, i think I have good news

I get still the same Errors when running the code in the real web application, but I have at least new inputs for evaluating the "Acces denied" issue.

First of all the 3 points you mentioned in your last answer:

1) different User account test consol app VS real web app:
a) The test console app is of course executed with my personel username that has admin rights
b) The web app (manges full with IIS) has in default the appl. pool account. I have changed this in IIS appl.pool advanced settings to LocalService, LocalSystem, NetworkService, and even to custom, my own username with which the test console app was running fine.
--> in each cases I got the same Errors when trying to browse servers and nodes
2) over the appl. pool I have also set for testing allow 32 bit exec. to true (default =false) --> again same errors.
3) For this topic I have nothing to add.
But now the good news*******************************************************************************

When I look to the eventlog I see that always the same Error is logged (independent how the user account for the web app is set)

OPCLabs-ComInterOp:

COM security initialization (process "w3wp" Id 14364, application domain "WebApplication7" Id 1) for requestor 'ComSecurityInitializingEasyDAClient' failed; the initialization object was: TurnOffCallSecurity=True.
CoInitializeSecurity failure (0x80010119): Security must be initialized before any interfaces are marshalled or unmarshalled. It cannot be changed once initialized.
+ This error (RPC_E_TOO_LATE) is not uncommon in hosted .NET applications. Depending on various factors, it might be possible to prevent it, or safely ignore it (if COM works as intended). Consult the product documentation.
+ Current thread name: ".NET ThreadPool Worker", from thread pool: yes, managed thread Id: 8, apartment state: MTA.

Reminding: I am using following parameters an these really at the top of my code:
ComManagement.Instance.Configuration.InstantiationParameters.OverrideDefaultSecurity = false;
ComManagement.Instance.Configuration.SecurityParameters.UseCustomSecurity = true;
ComManagement.Instance.Configuration.SecurityParameters.TurnOffCallSecurity = true;

I hope that this can help us forward...

Hello again
I have just tested the same code this time with a minized C# console app on .NET6. (Not web app)
Again after some trials I could browse servers , browse nodes and read OPC values with just following parameters:

ComManagement.Instance.Configuration.InstantiationParameters.OverrideDefaultSecurity = false;
ComManagement.Instance.Configuration.SecurityParameters.UseCustomSecurity = true;
ComManagement.Instance.Configuration.SecurityParameters.TurnOffCallSecurity = true;

On the real web app I get with this parameters again the same errors when browsing nodes (see below)

I agree that the problem could be the different user account on the web app.
Is the user account on the minimized .NET6 consol app "SYSTEM" ?
Is there a way to connect in the web app with the same user account on the console app?


Error for Browsing Nodes*********
Browse_node_Error The COM server does not support the interface 'OpcRcw.Da.IOPCServer'.

This problem could be caused by:
- incorrectly installed proxy/stubs.
- problems with the DCOM security settings.
- a personal firewall (sometimes activated by default).

+ The client method called (or event/callback invoked) was 'BrowseNodes'. 11/22/2023 4:36:37 PM

First of all thank you for your patient. I don't want to take your time, sorry in advance.
The first point I have understood .NET Framework and .NET use different implementations and cannot be compared in this manner.

But in my real application in .NET7: How can I overcome then the Error
"The COM server does not support the interface 'OpcRcw.Da.IOPCServer'"
in this case? Why do I get this Error? This is the point i didn't understood.

Hello

For(1)*************************
In the minimized test console app (1) we have .NET Framework 4.7.2. Here we have set Target platform in Visual Studip as "Any CPU" .As our server where the runtime is working is 64bit we can say that is working as 64-bit. (In the consul app there is also a checkbox like "Prefer 32-bit". We have tested with checked and unchecked. In both cases browsing branches and reading items worked fine.


For(2)*************************
In the real app we have .NET7. Here also we have set Target platform in Visual Studip 2022 as "Any CPU" .As our server (the same server like (1) where the runtime is working is 64bit we can say that is working also as 64-bit. But for Blazor server side there is no option box for "prefer 32-bit".