yeah, I've had this problem too for ages now. Latest license, dongles directly into computer. OSX 10.5.7 MacPro 2x3GHz quad cores, 20GB RAM
It seems to be fairly random, but regular.
I get a white square appear usually behind the ve window which if i leave for long enough (an hour or more) it will come up with an LCC error that one of the libraries cannot be found, syncrosoft error, update lcc.
Often I get this after closing 1 project in logic and trying to open another, or opening a project with more than 1 instance of ve.
I used to reboot, but it seems that if you quit all vienna related software in activity monitor, it will work fine 90% of the time when you restart the apps, and the rest it will find no licenses at all, so repair and reboot is necessary. But it is annoying. Seems to be worse when a deadline is approaching!