Rescan not creating artwork thumbnails

I'm on Logitech Media Server Version: 7.7.2 running on a Synology Disk Station (DSM 5.0). Everything has been great for a long, long time. Lately though my cover artwork has gotten weird: missing thumbnails of different sizes, wrong artwork, etc...

It's affecting the images on all devices: web, SB Touch, Squeezeplay, iPeng

I've tried clearing the library and rescanning from the web interface numerous times ("Clear library and rescan everything"). It updates information in the DB, but doesn't generate the missing thumbnails.

Is there a way to get thumbnails to regenerate? Or is there a way to flush the image cache so a rescan will create all new images?

Thanks in advance...
Matthew