@takti You need to understand that most people in this forum are volunteers spending their own time to help others for free. This includes me as well. Since I don't have a solution for your problem I suggested to check somewhere else also.
It is indeed strange. I haven't worked with that library, but I see no good reason that is should work with VlcMediaPlayer but not with VlcMediaListPlayer. In this example (graciously provided by @mrjj over chat), they don't seem to use VlcMediaListPlayer and as you said that should work. Perhaps you can use it to build upon. Also you might want to search through the library's bugtracker to see if there is something known about this issue.
Indeed, debug build should work as well. Hmmm, try switching to Debug, and do Clean All, run Qmake, Rebuild All. Maybe do that twice for good measure.
Anyway, now you're up and running in Release mode, happy VLC hacking!