These are my observations:
As mentioned before, the folder is a different one: "TDU\Data_DFE" instead of "TDU\Euro\Bnk\IGE".
The files have different names too: The name's are like "data_dfe_[16bit hex number]" with no extension. For example, the file for "TFE0045.IGE" (which should be "A Cute Little Race between Friends") is called "data_dfe_dbea71f8a00382d277a4244d873052f7".
The car restriction modification on "The Millionaire's Challenge", which i did yesterday, was gone today. Also, i can replace "TFE0045.IGE" with your version, but when i start the game, it is reset to the original race. It seems that when i start the game all modifications in the TDU\Data_DFE files are reset. Hex editing a file works as long as the game is running, but as soon as i quit and restart the game, the file is reset and the modifications are gone.
When i set the folder Data_DFE to read-only and start the game, i get a Securom message and the game does not start at all. When i remove the read-only flag all is working again. But, as mentioned before, all races are reset. I tested it online and offline, with the same result.
So, it seems that on my version of TDU some ingame code checks and/or resets the race files on game start. It's a totally legal copy, of course. Never had any problems with mods so far. All mods (Environment, Cars, Magic Map, Bling Bling etc.) are working, but somehow the races don't.
Still, thanks to your tutorial, i know how to do races like "The Millionaire's Challenge" under more difficult conditions, for example slower cars. That's the more important thing to me.
I let you know when i find something new about the issue.