I prepared simplest project that replicates the error. If I run the app outside the network with OPC server I get "RPC server is unavailable" in ServerDA.ReadItem. It is fine.

When run in correct network I get mentioned Error: Access is denied. Matrikon Explorer is able to connect/browse/display data on given OPC DA server.

VB6 project in the attachement.

In VB6 application I try to connect remote DA OPC server. In .NET application I'm setting ComManagement.Instance.Configuration.SecurityParameters.EnsureDataIntegrity = true; and calling ComManagement.Instance.AssureSecurityInitialization();, so there is no problem with server connection.

In VB6 app I'm getting following error when trying to read a tag value:

+ COM security initialization exception: System.Runtime.InteropServices.COMException (0x80010119): 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: "OpcLabs.Lazy2.Kick < [1] ", from thread pool: no, managed thread Id: 5, apartment state: MTA.
+ The machine name was 'TL-SIL-W2KS01'. The server class used was 'OPC.SimaticNET'.
+ Execution details: HasNativeClient=False, HasNetApiClient=True.
+ The client method called (or event/callback invoked) was 'ReadMultipleItems[1]'.

Is there any equivalent of setting EnsureDataIntegrity = true for Visual Basic 6 application?