TYPO3 Core

Did you read what the exception message contains?

Nesting type=inline in container sections is not supported. Throw an exception if configured.
Nesting sections is not supported. Throw an exception if configured.

Nesting flex form sections or inline within section containers is not supported.
To not let developers run into hard to track issues the patch adds an exception to detect these cases.

Additionally, GroupElement contains a preparation that is done by TcaGroup data provider already,
so this part is cleaned up along the way since this is unreachable code if FormEngine is used without nested sections.

Working with inline in section or with nested sections can result in various data integrity issues. Removing that exception would expose the system to that risk again. Core must stay on the safe side here.

This issue can only be closed at the moment.