I am consistently not able to open attachments from Group Notifications. If the group notification was sent while you were offline, this occurs. IF you are online and open the notification you just received, it will work as expected.
Sometimes it will display that the object has been saved to my inventory, however it does not. You can go into the Group Notifications in the Group floater to open it that way as a workaround.
This issue is present in all viewers I have tried, and research I have gathered from other third party viewer devs indicates this is most likely a server-side issue.