I updated to 0.6.7 today from 0.6.6. Turned the caching off, deactivated the plugin, updated to the new plugin, reactivated, then turned full caching and supercaching on. Back to normal.
I noticed then that I had some expired cache files, so I decided to delete them (and the old non-expired files) through the plugin controls just to have a fresh start. The expired regular cache files deleted fine, but the expired supercache files will not delete. I have 220 stacked up and no way to delete them via the normal method.