I had the same problem again earlier today.
The only temporary solution was, again removing all preferences (of all Cubase installations 5 ==> 7.5), run 7.5, quit 7.5 and then putting all preferences back.
Unfortunately, the whole GUI has been acting strangely since.
Faders not following automation, although the automation can be heard (even the faders on my Houston didn't follow), plugins not showing their GUI (or only a part of it), random plugins not loading (and no problem loading them after a reboot), channels in the channel edit view showing up too wide, not being able to colour tracks from the main colour menu (only on alt-clicking the track), the GUI of inserts or sends not showing up directly after they have been added to a project and, of course, random crashes after any one of these occurenses....
It seems to be an issue with the GUI not updating properly.
|MSI X58 Pro Intel i7 4096 Mb Ram|RME Fireface 800+Steinberg MR816x+ART TubeOpto 8|Cubase 7.5.20 |Windows 8.1 32-bit|Korg Microkontrol|Steinberg Houston|