Hey Lars,
i have the same behaviour at the moment. I can remember that i’ve had the same problem after one of the 6.0.x updates. Then I deleted the XML-files from the cubase6 appdata folder that contain the vstplugin-information and reinitialized all my plugins again. After that the problem was solved. I’ll give it a try with cubase 6.5 when I’m @ home again. Perhaps it solves the problem with 6.5, too 
Greetz,
Mike