This is the culprit: object x = _client.ReadItemValue( _Machine, _Server, _Tag );
where
private readonly EasyDAClient _client = new EasyDAClient();
using OpcLabs.EasyOpc.DataAccess;

Windows 10, Visual Studio Enterprise 2019, Winforms, c#

Server is Kepware (Kepware.KEPServerEX.V6) running on the same computer

Searches at OPC Labs for the error code (0x800401F3) have not proven very helpful

The Exception is:
An OPC operation failure with error code -2147221005 (0x800401F3) occurred, originating from 'OpcLabs.EasyOpcRaw.DataAccess.RawEasyDAClient' and with depth of 1. The inner exception contains details about the problem.

Inner Exception is:
Invalid class string + The client method called was 'ReadMultipleItemValues'.

In the call to ReadItemValue:
_Machine has been various values {e.g. "","localhost", actual machine name, etc.}
_Server is always Kepware.KEPServerEX.V6
_Tag is a tag known to exist in the Kepware server (e.g. XT1-P2)

Kepware is running fine with the Kepware tools showing usual stuff

Using a server simulator, the c# code has proven to be functional