After trying out the interface for a while, i finally found the values that seem to satisfy the API. Here is a code snippet:
blendfile = "D:/path/to/the/repository.blend"
section = "\\Action\\"
object = "myaction"
filepath = blendfile + section + object
directory = blendfile + section
filename = object
Hint: The directroy string must be terminated with a trailing "\\". If you do not do that, then you always get an error "not a library". I also found that you only need to supply filename and directory, which makes some sense:
bpy.ops.wm.append(filename=filename, directory=directory) # works
However this does not work, no idea why:
bpy.ops.wm.append(filepath=filepath) # does not work (why that?)