After migrating to another server, the downloadmanager plugin suddenly didn’t show anything but the categories. The data were still available in WordPress, only they weren’t showing on my page any more. This happens to many plugins after a server migration: nextgen, contactform7, wp-downloadmanager. Reason is the config data are serialized and after you change the paths of your mysql dump the data are no longer correct. The solution is to manually copy all relevant data from the old source to the new one, in this case: copy all empty Download Templates and change the path manually.