TYPO3 Core

The problem is the xml in database field pi_flexform.
I'm not exactly shure why this xml is in the field but I think it was a mistake during the convertion from grid_elements to container.

I can give you access to the system.

This is the xml:

<?xml version="1.0" encoding="utf-8" standalone="yes" ?>
<T3DataStructure>
    <meta>
        <langDisable>1</langDisable>
    </meta>
    <sheets>
        <sDEF>
            <ROOT type="array">
                <TCEforms>
                    <sheetTitle>LLL:EXT:container_elements/Resources/Private/Language/locallang.xlf:options</sheetTitle>
                </TCEforms>
                <type>array</type>
                <el type="array">
                    <containerClass type="array">
                        <TCEforms type="array">
                            <label>LLL:EXT:container_elements/Resources/Private/Language/locallang.xlf:container.class</label>
                            <config type="array">
                                <type>input</type>
                                <eval>trim</eval>
                                <size>30</size>
                                <valuePicker>
                                    <mode>append</mode>
                                    <items type="array">
                                        <numIndex index="0" type="array">
                                            <numIndex index="0">LLL:EXT:container_elements/Resources/Private/Language/locallang.xlf:container.class.content</numIndex>
                                            <numIndex index="1">container </numIndex>
                                        </numIndex>
                                        <numIndex index="1" type="array">
                                            <numIndex index="0">LLL:EXT:container_elements/Resources/Private/Language/locallang.xlf:container.class.cardGroup</numIndex>
                                            <numIndex index="1">card-group </numIndex>
                                        </numIndex>
                                    </items>
                                </valuePicker>
                            </config>
                        </TCEforms>
                    </containerClass>
                    <containerAttribute type="array">
                        <TCEforms type="array">
                            <label>LLL:EXT:container_elements/Resources/Private/Language/locallang.xlf:container.attribute</label>
                            <config type="array">
                                <type>input</type>
                                <eval>trim</eval>
                                <size>30</size>
                            </config>
                        </TCEforms>
                    </containerAttribute>
                    <isContainerFluid type="array">
                        <TCEforms type="array">
                            <label>LLL:EXT:container_elements/Resources/Private/Language/locallang.xlf:container.isContainerFluid</label>
                            <config type="array">
                                <type>check</type>
                                <default>0</default>
                            </config>
                        </TCEforms>
                    </isContainerFluid>
                    <wrapSection type="array">
                        <TCEforms type="array">
                            <label>LLL:EXT:container_elements/Resources/Private/Language/locallang.xlf:container.wrapSection</label>
                            <config type="array">
                                <type>check</type>
                            </config>
                        </TCEforms>
                    </wrapSection>
                    <sectionClass type="array">
                        <TCEforms type="array">
                            <label>LLL:EXT:container_elements/Resources/Private/Language/locallang.xlf:container.sectionClass</label>
                            <config type="array">
                                <type>input</type>
                                <eval>trim</eval>
                                <size>30</size>
                            </config>
                        </TCEforms>
                    </sectionClass>
                </el>
            </ROOT>
        </sDEF>
    </sheets>
</T3DataStructure>

If you have that xml in the pi_flexform field that is wrong. I guess we can properly guard the array key access error,
but that would still lead to an 503 for other things or produce any other weired issues.

You should properly clean these invalid xml from the database/pi_flexform field.

Patch set 1 for branch 12.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/81585

Patch set 1 for branch 11.5 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/81586