Howdy @Fabian
Welcome to the 
Community!
First can you confirm in the dropbox UI that the folder and file exist?
In the situation that they dont exist (click to view)
Depending on how you retrieve the file, you might need to use this module after the call to get the file from the database.
and input the contents of that to dropbox.
Most Data storage integrations on make dont accept a url as the actual file, you have to link the file contents specifically
Also Can you share a top level view of your entire scenario here?