Since OPC is based on COM and DCOM (Distributed COM), the error codes reported by OPC are usually hard-to-understand opaque COM error codes 0x8000????.
A common situation is that the OPC server is running locally on the machine, but SeqZap can still not connect or find the server (called browse the serer in OPC terminology).
A common way to fix this is to by-pass much of the DCOM permission system and connect directly to the local OPC server by setting MachineName to the empty string, instead of using localhost.