MagicMirror² v2.15.0 is available! For more information about this release, check out this topic.
SOLVED Lost resolution after reboot?!?
Man, I could freak out: The HDMI to DVI cable is somehow broken. I replaced it and everything works fine again. I spent hours to solve the problem… F**K!
Oh bad. Good you found it and good it was not fault of Windows … errm Raspbian