It is not possible to add an element of type @inline@ inside an element of type @inline@ in the *page view*. An "error 500" message is showing up. The logge error message is: > Core: Exception handler (WEB): Uncaught TYPO3 Exception: #1446996319: Configuration retrieved from FlexForm is incomplete or not of type "inline". | UnexpectedValueException thrown in file /var/www/html/typo3_src-7.6.21-bi/typo3/sysext/backend/Classes/Controller/FormInlineAjaxController.php in line 912. The same error occurs while trying to expand a collapsed inline element inside an other inline element. The error is only present in the page view. If I edit the "upper" inline element in list view, there is no error. It's the same problem described in #71436 and the patch in https://forge.typo3.org/issues/71436#note-49 works. More detailed description to reproduce the error: There is a plugin where I can add elements of type "inline" (in backend). These elements are the parent elements and contain a FlexForm definition. In that FlexForm area I can add some child elements (also of type "inline"). So when I edit my plugin and add some parent element and I try to add a child element, the error occurs. If I save my plugin (with the added parent element) and change to the list view, I am able to edit the parent element directly and add a child element. The error did not exists in TYPO3 6.2.