I am not sure what I am doing wrong.
Its generally a simple process. Create Usergroups, assign users to the usergroup. Create Jdownload Category, set Download as Allowed. But the user still does not have permission to download the files.
A few things I tried.
1) Changing the root usergroup. The custom ones I create were under the "Registered" a parent and now are under "Public" as parent. This made no affect.
2) Made sure that all files were inherit permissions for Download
3) I want the Public group to be Denied so the downloads are only for selected groups.
I'm using Joomla 3.6.2 and Jdownloads 3.2.45
[gelöscht durch Administrator]
I figured out the issue. So if anyone has the same issue as me. Make sure that the Public permissions for JDownloads component (JDownloads -> Options) is Inherited and Public permissions for any category is also Inherited. When I done this, the calculated settings says Not Allowed (Inherited). This then allows me to select any of my custom usergroups and successfully select "Allow" and the calculated setting also says Allow by the Download option. Hope this helps.
See also the documentation at
http://www.jdownloads.net/documentations/category/access-controls-and-user-group-permissions
Colin