Esri.arcgis.server exception from HRESULT: 0x80040111 (CLASS_E_CLASSNOTAVAILABLE) in GISServerConnectionClass()

I am getting following error from my DLL when executing following statement with ArcGIS Server 10.8:

IGISServerConnection conn = new GISServerConnectionClass();

Am I doing something wrong, following are some other information which may help you understand the issue:

1. I am using .NET Framework 4.6.1
2. Marked Embed Interop Type = false for reference esri.argis.server assembly.
3. calling the dll through a web-handler (.ashx) file.
4. in IIS 7.5, default application pool enable 32 bit application as true.
5. All assemblies are registered in the machine where I deployed this dll, I also registered this dll using esriregasm.exe
6. Esri.arcgis.server.dll version is 10.8
7. Esri server manager 10.8 is deployed on Apache

Would be great if anybody can help in resolving this issue. Many thanks in advance.