sometimes I get this error on saving the settings, sometimes on plugin install (after renaming the plugin to get my site back. Looks like the quotes should not be included.
Ai1ec_Settings::get_instance() [<a href='ai1ec-settings.get-instance'>ai1ec-settings.get-instance</a>]: The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "Ai1ec_Settings" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /nas/wp/www/cluster-1173/taramandala/wp-content/plugins/all-in-one-event-calendar/app/model/class-ai1ec-settings.php on line 299
using version 1.8.2. the problem also occurs with version 1.8.3