Microsoft Windows defines the standard DCOM error messages. DCOM errors typically begin with a "0x800" code. Classic OPC (previous to OPC UA) uses DCOM as its transportation mechanism. Thus, OPC states what information must transfer, and DCOM handles the transfer itself. DCOM also handles security aspects such as authentication and encryption.
There are a multitude of reasons for an OPC Client application to fail to connect to an OPC Server due to DCOM problems. The OPC Training Institute’s website covers many of these reasons in detail. Our whitepapers (available for free) list various error conditions, their causes, and how to solve them using a step-by-step guide.