Update: Still no resolution to this issue. I have sent emails and created support tickets with GOG, Stardock, and Ironclad. Thus far Stardock is the only one who has responded to my emails, and they say everything is good on their end - my username and email are correct, and we even reset my password, but I am still getting the error. I suspect that only Ironclad can help me since ICO is run by them.
I have tried looking through the game files, through the .ini settings, and anything else that I can think of, but am still at a loss. In game there is no options for account settings - no name, password, email address, or other credentials, no game/product key, nothing at all. So I don't understand how I can get an "Account name not found" error when there is no way to enter or submit that info. I simply click on the "ICO Internet Muliplayer" button on the main menu and it immediately gives me the error. If I run the game through GOG Galaxy and try connecting to ICO then it gives me the option to input a name, but it won't allow me to use my name "Onsdag" as it is already taken. So somehow the game files must have been changed with the newest updates, or something.
Does anyone have any other suggestions? I feel like I'm having a one-sided conversation here. Aren't these the forums for technical support and help with the game?