Stack Trace:     System.Web.Services.Protocols.SoapException: System.Web.Services.Protocols.SoapException: System.Web.Services.Protocols.SoapException: Server was unable to process request. ---> System.Runtime.InteropServices.COMException: Unknown error (0x80005000)
at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail)
at System.DirectoryServices.DirectoryEntry.Exists(String path)
at SolidCP.Providers.RemoteDesktopServices.Windows2012.CheckOrCreateComputersRoot(String computersRootPath)
at SolidCP.Providers.RemoteDesktopServices.Windows2012.MoveSessionHostToRdsOU(String hostName)
at SolidCP.Server.RemoteDesktopServices.MoveSessionHostToRdsOU(String hostName)
--- End of inner exception stack trace ---
at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
at SolidCP.EnterpriseServer.RemoteDesktopServicesController.AddRdsServerInternal(RdsServer rdsServer)
at SolidCP.EnterpriseServer.esRemoteDesktopServices.AddRdsServer(RdsServer rdsServer)
at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
at SolidCP.EnterpriseServer.esRemoteDesktopServices.AddRdsServer(RdsServer rdsServer)
at SolidCP.Portal.RDSServersAddserver.btnAdd_Click(Object sender, EventArgs e)

With the help of the following post

https://solidcp.com/forum/question/unknown-error-0x80005000-when-selecting-a-site/page/2/

I fixed it by checking the Active Directory Connection settings under the server with the Remote Destop Services role.