TYPO3 Core

Could you please post the accordant TCA definition of that field?

Within the table tx_seminars_seminars:

'prices' => array(
            'exclude' => 1,
            'label' => 'LLL:EXT:seminars/locallang_db.xml:tx_seminars_seminars.prices',
            'config' => array(
                'type' => 'inline',
                'foreign_table' => 'tx_seminars_prices',
                'foreign_field' => 'seminar',
                'maxitems' => 999,
            ),
        ),

And here's the problematic part within tx_seminars_prices:

'currency' => array(
            'exclude' => 1,
            'label' => 'LLL:EXT:seminars/locallang_db.xml:tx_seminars_prices.currency',
            'config' => array(
                'type' => 'select',
                'internal_type' => 'db',
                'allowed' => 'static_currencies',
                'foreign_table' => 'static_currencies',
                'size' => 1,
                'minitems' => 0,
                'maxitems' => 1,
                'items' => array(
                    '' => '',
                ),
            ),
        ),

I've uploaded a test extension. Try reproducing by creating a "Parent" record and click on the "Create new record" link to create new "Price" IRRE relation.

I think that when responding to an IRRE Ajax call while the backend is not utf-8-encoded, the data should be utf-8-encoded before submitting it to json_encode.

yup. this is now handled in the TYPO3 core. Can you check if this bug still exists then?