Hello,
some thoughts on this:

1. We do not support Microsoft Windows Server 2008 R2 Enterprise in QuickOPC 2018.3. I think it likely is not the cause of the problem, but you should be aware of it.
2. I believe that the server should never return text/html, for SOAP requests. If the server encounters an error, it should still return text/xml, with proper error indication as specified by OPC XML-DA.
3. The error seems to be related to the GetStatus method call. We are (periodically) calling the GetStatus method. It is possible that the dOPC Explorer does not do it (although a compliant OPC client is required to do so), and that the method is not implemented or has a bug on the server side. We have a setting that should disable the GetStatus method call. I will send the necessary instructions in next post here. It might help (or not...)

Best regards

Hello.

1. Are you sure you are connecting to the right URL? What is the URL of the server you are using?
2. is the server on private network, or publicly available?
3. If on private network, can the endpoint be made available to us (open the firewall for a specified source IP address), so that we can connect from our side, and diagnose?

Thank you

Hello,
can you please provide additional details?

1. Which version and build of QuickOPC are you using?
2. Is it under .NET Framework or .NET Core?
3. Which operaring system and version are you running the client on?
4. What is the target OPC server (vendor, type).

Thank you