I don't have an OPC server to test with, but it does work with one of the native drivers, so it's something in the OpcDaCom driver. Does it give you any indication of which line number the error occurs?
Open OpcDaCom.vb and put a break point on line 209 by clicking in the left margin. Run the app, click the button and when it stops at the break point, step through with F10 until you get the error, then let me know what line the error occurred.