In my case (typo3 10.4) I told the list plugin to load the detail view at a separate page but forgot to add the detail plugin at this page. My error message has been: No Content Object definition found at TypoScript object path "tt_content.list.20.9" in typo3_src-10.4.37/typo3/sysext/fluid/Classes/ViewHelpers/CObjectViewHelper.php:165