Ok, that explains it, although I really don't understand, why ModX introduces media sources as a whole new way to handle files and recommends not using the old properties for the filemanager-paths/URLs in the system settings, if another basic feature like static resources rely on the old method.
So as I'm forced to select a file from a media source for a static resource from within the manager, I selected the default "file system" media source (which points to the root of my ModX installation, just like the filemanager system settings) and then the file. Still no luck, still only get 0-bytes-sized files and no error messages...
referenced file for static resource:
assets/media/downloads/12.pdf
URL for static resource used for links:
http://site/de/produkte/produktpalette/palblend-pc-abs-blends/palblend-r-45.pdf
("de" used as key by babel extension)
system settings' filemanager_path: empty
system settings' filemanager_path_relative: yes
system settings' filemanager_url: empty
system settings' filemanager_url_relative: yes
Does anyone see a mistake I made here or has a solution for my problem? Any help will be greatly appreciated. Thanks!