TYPO3 Core

\TYPO3\CMS\Backend\Form\FormDataProvider\AbstractItemProvider->buildForeignTableQueryBuilder()

calls

\TYPO3\CMS\Backend\Form\FormDataProvider\AbstractItemProvider->processForeignTableClause()

which returns an array with the processed query parts from the foreign_table_where.

The GROUPBY part is an array, since the function explicitly explodes the GROUP BY fields.

This is then passed on to

if (!empty($foreignTableClauseArray['GROUPBY'])) {
    $queryBuilder->groupBy($foreignTableClauseArray['GROUPBY']);
}

where it all goes wrong. This method uses the variadic operator "..." for $groupby, which contains remaining (in this case all) arguments as an array. So now we have a multi-dimensional array, where the ensuing code expects a flat array. When it attempts to quote identifiers, it ends up passing an array to strpos(), consistently causing a PHP error.

The end-result is that any attempt to create or edit a TCA record that contains such a foreign_table_where on one of its columns or via an inline record configuration, results in an uncaught TYPO3 Exception.