It works even though that message is displayed. But now I see two more error messages in settings > excerpt when saving:
Notice: Undefined index: advancedexcerpt_no_custom in …/wordpress/wp-content/plugins/advanced-excerpt/advanced-excerpt.php on line 262
Notice: Undefined index: advancedexcerpt_finish_sentence in …/wordpress/wp-content/plugins/advanced-excerpt/advanced-excerpt.php on line 265
The error messages are not displayed if the boxes are checked in settings > excerpt.