Date: 03/02/2013 Enhanced Folding Category List for WordPress 2.8+
Try deleting any unused widgets from the "inactive widgets" section of the widgets dashboard and try again.
WordPress 2.8+ Simply drag the focal icon to your sidebar(s) as needed.
WordPress 2.3 - 2.7 Below your available widgets section, you should see a section asking "How many Folding Category widgets would you like?". Simply change the number in the box and click save. For some reason it does not always add extra widgets on the first attempt, but it works fine the second time.
You need to Rebuild the Cache for changes to take effect.
Data structures have changed to increase performance and may be incompatible with the latest version. Please Rebuild the Cache to resolve the errors. You need only do it once.
Ensure that "Automatically rebuild cache" is enabled or you manually click "Build Cache".
Allowed memory size of 33554432 bytes exhausted (tried to allocate 274388 bytes)
In some rare cases WordPress is unable to update its internal cache object and crashes out. Please see http://wordpress.org/support/topic/161173?replies=3 for progress on this issue.