It could be an "out of memory" error. You can confirm by temporarily adding this to wp-config.php:

define( 'WP_DEBUG', true );

first, you can try adding this to your wp-config.php:

define('WP_MEMORY_LIMIT', '48MB');

If that doesn't work, your web host must be imposing a lower php memory limit. Disable Role Scoper. If you are unable, try adding this to wp-config.php first:

define('RS_DEBUG', true);

To get a better handle on your memory usage and limits, install the TPC Memory Usage plugin and experiment with temporarily deactivating various plugins. Once you free up more memory or increase your limit, reactivate RS.