why is cubase suddenly always saving images/freezes/soundfiles under device “C:” on Windows 7 under “unknown1,2,3…” Folders???
Because this is your defined default folder, and you didn’t create new folders for new projects. Create a different default folder, or create individual folders for each new project.
I always make sure the folder structure is correct in the beginning of a project. Since 6 Years I always had those.
That’s why I counted on the audio/peaks/and the others to be there too. But in my project folder those folder weren’t made and Cubase decided to make them in C: suddenly since the latest two projects. But what has changed to make it behave like that? I’m sure I didn’t do anything different since January. 
Even manually changing those folders to the main folder of the project and letting it find the sounds there doesn’t change much. Next time the peak folder appeared again in C: users.
Maybe It’s time to install the system anew again - I always had troubles after updates without reinstalling the whole computer.
From file menu select new project.
In the hub/assistant window that opens there at the bottom is prompt for project location selected?
Grim, thanks a LOT. 
Indeed it wasn’t selected - I didn’t even know such a selection/function is there.
I’ll have to keep an eye on that one, as I’m used to use a starter project from which I save a new version via template.