Yes, in general with OPC you should never have the servers or OPCEnum set to run as "Launching user". This becomes a problem because the clients may run under different accounts, which you cannot generally control, so the behavior would differ depending on the user account of the client. In addition, when clients connect with different accounts, multiple instances of OPC servers or OPCEnum will be started - which is usually quite bad to have. "Interactive user" is not a good setting either. It should always be a pre-set, specific account, as you did.

But setting the account under which the OPC server or OPCEnum runs is just a part of the DCOM configuration. What becomes important then is the access permissions - you need to review them and add the accounts needed. And, access permissions that are set on specific COM components (OPC servers, OPC Enum) are further restricted by global DCOM "limits", that are configurable in DCOMCNFG on the machine level (right-click the computer node). Even if you allow an access to a specific account on an OPC server level, it won't work unless the DCOM "limits" also allow that account. And, sometimes, changing the DCOM security settings only takes real effect after computer restart. Terrible stuff to work with.

You can also try to assign different combinations to following static properties - in the code, before the very first OPC operation is attempted:

EasyDAClient.ClientParameters.UseCustomSecurity
EasyDAClient.ClientParameters.TurnOffActivationSecurity
EasyDAClient.ClientParameters.TurnOffCallSecurity

I have checked the precise meaning of the original error message, i.e.

System.Runtime.InteropServices.COMException (0xC0049039): No method available to convert ProgID to CLSID
ErrorCode: -1073442759

It is issued when there is a problem connecting to the (local, in this case) OPCEnum service (which also serves the purpose of converting ProgIDs to CLSIDs), and the ProgID cannot be converted by other means either. It looks like a security issue. When you make the DCOM settings, do not make them just for the OPC server, but also make the same for OPCEnum.

I also wanted to suggest using the CLSID directly; it is weird that it does not work. When you specify the CLSID, please make sure you put it in curly brackets, e.g. "{C8A12F17-1E03-401E-B53D-6C654DD576DA}" for our simulation server.

The error may have to do with accessing OPCEnum component.
Are you connecting to an OPC server on a remote machine?