Super Cache

Hello,

We run several WP sites that use SuperCache, and have upgraded many of them to 0.9.9. One of them seems to have stopped working entirely. Pages are not stored in the cache folder (the cache folder contains nothing but .htaccess and an empty meta folder), and the footer has the following error message:
<!-- Page not cached by WP Super Cache. Could not get mutex lock. -->

I’ve tried $use_flock = true, I’ve tried changing the value of $sem_id, and I’ve tried including umask( 0022 ); in wp-config.php. I also have tried regenerating the .htaccess rules, just in case. So far, none of these have helped, and the tester continues to say that the timestamps don’t match.

I haven’t been able to find any other things to try, and would love to hear if there are other things I could try.

Thanks,
Jonathan