Folders and files in Dropbox folder work with some sort of additional script in the background.
I realised it when I checked in "Auto-save" and C4D poped up a window with information, that the file cannot be written.
Also Photoshop doesn't like this folder to auto-save or save with scripts, and get's crashes.
You may encounter problems even when you turn off Dropbox, but work on files/folders, that are in Dropbox folder.
Solution: take your project files out of Dropbox folder and you're good to go.
Mystery solved!
thanks Aoktar!