This is a common problem with USB interfaces (and why I prefer FireWire ).
If disconnected, then plugged into a different USB port, Windows will treat it as a "new" device and re-name it.
Since Hauptwerk can only respond to what Windows tells it is present or absent, you get those error messages.
I frequently use several different interfaces for testing on the same PCs, and to avoid having to reset all the MIDI assignments every time, I change all the MIDI Inputs and Outputs to "Any enabled port" rather than a particular interface name. I think I'm right in saying that this shouldn't cause any sample re-caching, since the audio routing and other options haven't changed.