flexform sheets are not modified during upgrade
TYPO3 plugins using flexform sheets do not work any more after an upgrade from TYPO3 8 to TYPO3 9.
The reason are the modifications in the flexform sheets. "s_general" has been stored as "general" in flexforms before TYPO3 9. With TYPO3 9 they must be stored as "s_general".
The upgrade wizards forgets to modify the flexform sheet name and add a leading "s_" to them.