Well, I have tried several things… even just open Cubase - no project, no nothing - after an hour I quit Cubase and it freezes. I have tried only with VST3 plugins, with 2 different audio interfaces, different projects, different plugins active and so on. I have deleted the Cubase Preferences folder. At least 8 out of 10 times Cubase freezes on quit. This was the case with Cubase 6.5.0 and it still is the case with Cubase 6.5.3.