My suggestions/questions were related to: 1 instance of OPC Server on 1 computer and the 2nd computer accessing that over network.
Your explanation makes things different now and suggests that you have the same independent setups, one on a laptop and one on a desktop computer. Could you suggest of how do you run the program (did you copy the Debug folder from laptop or did you add your project to 3.98t version...)?
Generally, the 1st error that you are getting, if it is 0x80070005, would point to DCOM Permission problems (slightly weird since it would be sort of network related and you are not using the network for your setup). The 2nd error is just a consequence of the first error (since there is no connection to the server it cannot find an instance of the object). Here is a link suggesting issues with .Net for similar error:
http://www.schneider-electric.us/sites/us/en/support/faq/faq_main.page?page=content&country=ITB&lang=en&locale=en_US&viewlocale=en_US&id=FA212076&prd=&redirect=trueIt is sufficient to Google your error and look at some results/explanations. Luckily, you have a setup that is already working for you so you could do things in parallel and compare laptop DCOM configuration with desktop DCOM configuration. Since the laptop is working then you should try to setup your desktop with the same settings (assuming it is DCOM issue).
I am using MatrikonOPC Explorer to find all OPC servers available on my computer and/or network computers and then choose one of them and connect to it. It is free to download and you can see a picture of it in this post
https://www.advancedhmi.com/forum/index.php?topic=648.
If this explorer cannot find your OPC server or connect to it then AHMI shouldn't be able to do so either.
If it does find your server that should indicate that your server was installed properly and OPCenum has enumerated it.
If it cannot connect to the server then it should pop an error message. Try it since it can only help you identify the cause.
One more thing to mention to you is to copy the whole AHMI project that you have on a laptop and use it on a desktop computer. If it works then there might be an issue with the OpcDaCom driver that is included in version 3.98t (which shouldn't really be the case since it works for me on my computer and over the network as well). From what I recall the driver has changed since 6 months ago and that's why I am suggesting to try the old AHMI version.