Ok, so I have tried several adjustments, both on the pc and the tv. I have a Samsung 4K tv set to "game mode".
This is probaly not a bug or an error, but the game is definately much more sensitive to settings affecting black levels, than most other games.
I changed the colour space on the tv (from the default setting "native" to "auto") and adjusted the brightness a bit up (from 50 to 55). That enabled me to use full RGB range without having the extreme darkness in shadow portions etc. in the game. The brightness adjustment screen in the game is a good reference. The leftmost Dirt-logo should be barely visible with brightness set to 1.00. If it isn't, something's off.
So I think my conclusion is that my settings were bad, but not nearly bad enough to manifest itself into problems in other games. My settings now look better altogether in all games, so this was a useful experience. 🙂