When item-list Markup Override is checked the following error is raised while calling theme('item_list'):

PHP Fatal error:  [] operator not supported for strings in /var/www/html/cm
7/sites/all/themes/adaptivetheme/at_core/inc/theme.inc on line 249, referer: http://www.xxx.com/cm7/de/admin/appearance/settings

The items are passed as follows:

    ... (Array, 7 elements)
        0 (Array, 2 elements)
            data (String, 258 characters ) <div class="weblinks-link"><a href="http://www....
                <div class="weblinks-link"><a href="http://www.plm.automation.siemens.com/de_de/products/tecnomatix/support/production_execution/fl_authorization.shtml" target="_blank" class="previewlink" title="FactoryLink Authorization">FactoryLink Authorization</a></div>
            class (String, 39 characters ) weblinks weblinks-item weblinks-link-40
        1 (Array, 2 elements)
            data (String, 224 characters ) <div class="weblinks-link"><a href="http://www....
                <div class="weblinks-link"><a href="http://www.plm.automation.siemens.com/en_us/products/tecnomatix" target="_blank" class="previewlink" title="Tecnomatix: Products and Solutions">Tecnomatix: Products and Solutions</a></div>
            class (String, 39 characters ) weblinks weblinks-item weblinks-link-38
        2 (Array, 2 elements)
        3 (Array, 2 elements)
        4 (Array, 2 elements)
        5 (Array, 2 elements)
        6 (Array, 2 elements)

    Krumo version 0.2.1a | http://krumo.sourceforge.net [Click to expand. Double-click to show path.] 
Called from /var/www/html/cm7/sites/all/modules/weblinks/contribs/weblinks_blocks/weblinks_blocks.module, line 164

The output is created by the following call:

  return $items ? decode_entities(theme('item_list', array('items' => $items, 'title' => NULL,
                                        'type' => 'ul', 'attributes' => array('class' => $classes))) . $more_link) : NULL;

When the item-list Markup Override is unchecked the list ist is displayed as expected.

Is this more likely a themeing issue or a problem how the list is setup?