TYPO3 Core

When I click on "Add image" and select a file in popup an alert box appears which says error 500. TYPO3 7.6 LTS.

This is from error log:

Core: Exception handler (WEB): Uncaught TYPO3 Exception: #1446996319: Configuration retrieved from FlexForm is incomplete or not
of type "inline".| UnexpectedValueException thrown in file /kunden/303809_70794/rp-hosting/12529/62529/typo3_src/master/typo3/sysext/backend/Classes/Controller/FormInlineAjaxController.php
in line 811. Requested URL: http://***.de/typo3/index.php?ajaxID=%2Fajax%2Frecord%2Finline%2Fcreate&ajaxToken=7f5400a984784016695d4bf812cd3d14f031d1c6    

The configuration of this field is:

<config>
    <type>inline</type>
    <foreign_field>uid_foreign</foreign_field>
    <foreign_label>uid_local</foreign_label>
    <foreign_match_fields>
        <fieldname>fal</fieldname> <!-- Replace "fal" with name of variable of this field (e.g. images)! -->
    </foreign_match_fields>
    <foreign_selector>uid_local</foreign_selector>
    <foreign_selector_fieldTcaOverride>
        <config>
            <appearance>
                <elementBrowserAllowed>gif,jpg,jpeg,tif,tiff,bmp,pcx,tga,png,pdf,ai,svg</elementBrowserAllowed>
                <elementBrowserType>file</elementBrowserType>
            </appearance>
        </config>
    </foreign_selector_fieldTcaOverride>
    <foreign_sortby>sorting_foreign</foreign_sortby>
    <foreign_table>sys_file_reference</foreign_table>
    <foreign_table_field>tablenames</foreign_table_field>
    <foreign_types type="array">
        <numIndex index="0">
            <showitem>--palette--;LLL:EXT:lang/locallang_tca.xlf:sys_file_reference.imageoverlayPalette;imageoverlayPalette, --palette--;;filePalette</showitem>
        </numIndex>
    </foreign_types>

    <filter type="array">
        <numIndex index="0">
            <parameters>
                <allowedFileExtensions>gif,jpg,jpeg,tif,tiff,bmp,pcx,tga,png,pdf,ai,svg</allowedFileExtensions>
                <disallowedFileExtensions></disallowedFileExtensions>
            </parameters>
            <userFunc>TYPO3\CMS\Core\Resource\Filter\FileExtensionFilter->filterInlineChildren</userFunc>
        </numIndex>
    </filter>

    <minitems>0</minitems>
    <maxitems>99</maxitems>

    <appearance>
        <useSortable>1</useSortable>
        <headerThumbnail>
            <field>uid_local</field>
            <width>45c</width>
            <height>45</height>
        </headerThumbnail>

        <showPossibleLocalizationRecords>0</showPossibleLocalizationRecords>
        <showRemovedLocalizationRecords>0</showRemovedLocalizationRecords>
        <showSynchronizationLink>0</showSynchronizationLink>
        <useSortable>1</useSortable>
        <enabledControls>
            <delete>1</delete>
            <dragdrop>1</dragdrop>
            <hide>1</hide>
            <info>1</info>
            <localize>1</localize>
            <new>0</new>
            <sort>1</sort>
        </enabledControls>
        <createNewRelationLinkTitle>LLL:EXT:frontend/Resources/Private/Language/locallang_ttc.xlf:images.addFileReference</createNewRelationLinkTitle>
    </appearance>

    <behaviour>
        <localizationMode>select</localizationMode>
        <localizeChildrenAtParentLocalization>1</localizeChildrenAtParentLocalization>
    </behaviour>
</config>