More Awesome Than You!

I'm running BV on a fairly new install.  Today, mysteriously, my game lost the ability to generate object thumbnails or sim portraits.  Precisely what's missing are:

- custom content object thumbnails (the Maxis objects show up correctly in the catalog, but any custom items show a blank square)
- inventory/leftover thumbnails
- houses in neighborhood view -- I can see floating stairs, but everything else is invisible.
- family portraits, both in the preview when you click on the house in neighborhood view, and on the family loading screen
- sim portraits -- the game retains the old ones fine, but when it has to regenerate a new one it throws a blank.  Both the sim thumbnail at the left of the screen and the "selected sim" portrait at the bottom of the screen are blank.  So are any wants related to that sim and that sim's face in the relationship panel.  It's creepy.

Messing with my graphics options/screen resolution has no effect; neither did removing all custom content or deleting groups.cache.  Any idea what the hell is going on?

Did you try deleting all the thumbnails files and letting them regenerate?

That certainly had an effect, but not the desired one.  :o  ALL thumbnails are now blank.  The game seems to be attempting to regenerate them (it lags briefly while loading catalog pages) but it has lost the ability.

Is the thumbnails folder writable?