TYPO3 Core

In a flexform field of an extension plugin I have defined an inline element for multiple FAL images. When I enable the button for adding a new element within the list

<enabledControls type="array">
    <new>TRUE</new>...

the button appears but a 500 server error pops up. Debugging in typo3/sysext/backend/Classes/Form/Element/InlineElement.php showed that the configuration is not properly taken for the file reference but instead mixed up with serialized configuration parts.

Probably this part is not fully implemented yet?
The normal addition of references works but if you want to add several items to the middle of a long list it's a bit annoying to move them from the end to the middle.

The full flexform:

<el> 
    <settings.images>
        <TCEforms>
            <label>Images</label>
            <config>
                <type>inline</type>
                <foreign_types type="array">
                    <numIndex index="2" type="array">
                        <showitem>--palette--;LLL:EXT:lang/locallang_tca.xlf:sys_file_reference.imageoverlayPalette;imageoverlayPalette,--palette--;;filePalette</showitem>
                    </numIndex>
                </foreign_types>
                <foreign_table>sys_file_reference</foreign_table>
                <foreign_field>uid_foreign</foreign_field>
                <foreign_sortby>sorting_foreign</foreign_sortby>
                <foreign_table_field>tablenames</foreign_table_field>
                <foreign_match_fields type="array">
                    <fieldname>image</fieldname>
                </foreign_match_fields>
                <foreign_label>uid_local</foreign_label>
                <foreign_selector>uid_local</foreign_selector>
                <maxitems>1000</maxitems>
                <foreign_selector_fieldTcaOverride type="array">
                    <config>
                        <appearance>
                            <elementBrowserType>file</elementBrowserType>
                            <elementBrowserAllowed>jpg,png</elementBrowserAllowed>
                        </appearance>
                    </config>
                </foreign_selector_fieldTcaOverride>
                <appearance type="array">
                    <headerThumbnail>
                        <field>uid_local</field>
                        <width>50m</width>
                        <height>50</height>
                    </headerThumbnail>
                    <createNewRelationLinkTitle>Add image</createNewRelationLinkTitle>
                    <useSortable>TRUE</useSortable>
                    <showPossibleLocalizationRecords>FALSE</showPossibleLocalizationRecords>
                    <showRemovedLocalizationRecords>FALSE</showRemovedLocalizationRecords>
                    <showSynchronizationLink>FALSE</showSynchronizationLink>
                    <enabledControls type="array">
                        <info>TRUE</info>
                        <new>TRUE</new>
                        <dragdrop>TRUE</dragdrop>
                        <sort>TRUE</sort>
                        <hide>TRUE</hide>
                        <delete>TRUE</delete>
                        <localize></localize>
                    </enabledControls>
                    <levelLinksPosition>both</levelLinksPosition>
                </appearance>
            </config>
        </TCEforms>
    </settings.images>
</el>

Using:
TYPO3 6.2.14
PHP 5.4.41-0+deb7u1