I have just gone back on TDU2 after not going on it for a year and I keep getting error 3, if I press retry, it just downloads some stuff and comes up with it again. There are game servers open and my NAT type is open full cone. I do have the game legitimately as I bought it off amazon.
try changing your NAT type it works for me on xbox sometimes when i have open like you my cods do not find game and because tdu is online test you connection or fiddle whit it - - - Updated - - - or try going to the website and finding a updater
Well the error actually means that the program doesnt have permission to open one of the files. Running as administrator usually fixes that as administrator can open all files, running a program under administrator also allows the program to access all files.
The publisher Atari went bankrupt, but the game servers are hosted by the developers Eden Games, as far as i remember ... so online mode should work. Gotta give it a try again soon