Helpdesk only came up with some standard answers so I uninstalled the app and downloaded some lectures again. It looks like they are downloaded (Switched off internet and they show). But can I watch them? 3 minutes and after that it skips to the next, the next and so on. The courses I try to watch are great but the app sucks and it doesn't look like the staff at Udemy is competent enough to solve the problems so maybe the course providers should consider to offer their courses somewhere else.