TYPO3 Core

Sounds like the same what we are experiencing currently.

We have a content element where editors can add one or more items to a list via a flexform.

If editors create a new ce, they can add as many items as they want. If the save it and then try to add a new item, it results in an error 500. Looking at the console, reveils an error 500. The exception message is

    No content edit permission for user 2 on page root node

This is "right", because the user does not have permission to edit on the root page. On the other hand, this is the explanation, why this does not "harm" admin users. They are allowed to edit records on uid 0.

The ajax request in both cases (new ce and saved ce) is:

http://t3trunk/typo3/index.php?ajaxID=%2Fajax%2Frecord%2Fflex%2Fcontaineradd&ajaxToken=628ecdf44b955861e2ba1b9c0ee2a98478cbfed6

I'm using the current master with php 7.0

Please see the patch https://review.typo3.org/#/c/52174/

Our current workarround to get this issue done for two new content elements is to add an own FormDataProvider. Thx to Markus Sommer for the quick fix.
Maybe someone else needs this.

ext_tables.php of the template extension:

$class = \In2code\In2template\Backend\FormDataProvider\FixEditorPermissions::class;
$GLOBALS['TYPO3_CONF_VARS']['SYS']['formEngine']['formDataGroup']['tcaDatabaseRecord'][$class] = [
    'before' => [
        \TYPO3\CMS\Backend\Form\FormDataProvider\ReturnUrl::class,
    ]
];

FixEditorPermissons.php:

<?php
declare(strict_types = 1);
namespace In2code\In2template\Backend\FormDataProvider;

use TYPO3\CMS\Backend\Form\FormDataProviderInterface;
use TYPO3\CMS\Backend\Utility\BackendUtility;

/**
 * Resolve return Url if not set otherwise.
 * This fixes FlexForm with inline elements. Editors can not add new itemsets.
 * An error 500 is thrown in browser console that's not allowed to add new items to page with uid=0.
 */
class FixEditorPermissions implements FormDataProviderInterface
{

    /**
     * @param array $result
     * @return array
     */
    public function addData(array $result)
    {
        if ($result['tableName'] === 'tt_content') {
            $result['databaseRow'] = BackendUtility::getRecord($result['tableName'], $result['databaseRowUid']);
        }
        return $result;
    }
}

I'm able to reproduce the issue with the extension and will review the proposed pending fix now.