These are actually two games (possibly more) in a series of adventure games. I remember one more than the other, so I'll be talking about that one first.
I played both games sometime between 2003 and 2008. The game used pre-rendered 'realistic' 3D for its presentation. I played it on a PC running Windows ME.
I played neither very far, as the puzzles were too complicated for me as a child. I also played it in German, so this might never have been released outside of Germany.
The protagonist is a boy/teenager who receives his 'next mission' on a VHS tape delivered by mail. The game makes use of full-motion 3D animation with voice acting for the intro video and also some short sequences throughout the game. The boy is sent to investigate a city that is mostly abandoned. One of the reasons for this includes reports of the drinking water turning into a green sludge.
The player would now explore the city from a first-person perspective, solving puzzles and such. Noteworthy is that there were basically no NPCs in the game. You mostly interact with the environment.
Here are some more details I remember from the first game:
- The one NPC I did meet was an old and sick man who gave the player an empty bottle with the instruction to get him some medicine. I'm sure there was a puzzle to mix the medicine.
- There was a robot guard dog that didn't let me through the gate into a courtyard. I think some sort of hacker character might have lived there, but I only remember this one from the box art.
- The city had a subway system. The player could control the train. One puzzle involved changing a rail switch to reach new destinations. Said puzzle involved a coloured tile appearing on a screen and the player having to match it to some input. When moving the train cart, a short first-person cutscene was played, which showed you the alternative path you just drove past.
- There was a fire station with a deactivated firefighter-robot. the robot looked similar to the one from the movie Number 5 Lives, AKA Short Circuit (1986). This robot was also featured on the box art.
A few details from the second game:
- This one takes place on an island. The mystery here involved reports about weather anomalies.
- There was a puzzle involving baking bread.
Outside of the game itself, I remember it being 'advertised' in a 5th grade school book. So this might actually be an edutainment game, but I wouldn't count on it. I got these two games as a birthday gift once, but my family gave them away because I couldn't get much out of them.
Update: I had a call with my grandma, who used to be a german teacher, and it turns out she has a copy of the school book I used in 5th grade. She will visit my parents home on thundery and bring it along. I'll be there over the weekend and check it out. Hopefully this will will give me a definite answer. So to anyone who wants to answer this question themself, you're on a time limit now ;)