RuslanSerr

Оказывается вместо Fetch() нужно было использовать GetNext()
Странно, в выборке поле "DETAIL_PAGE_URL" отмечено
Итог

<?
        $arSelect = Array("ID", "IBLOCK_ID", "NAME", "PREVIEW_PICTURE", "DETAIL_PAGE_URL", "PROPERTY_NABOR");
        $arFilter = Array("IBLOCK_ID"=>$arParams['IBLOCK_ID'], "ACTIVE"=>"Y", "PROPERTY_NABOR"=>$arResult['DISPLAY_PROPERTIES']['NABOR']['VALUE']);
        $res = CIBlockElement::GetList(Array("SORT"=>"ASC"), $arFilter, false, false, $arSelect);
        ?>
        <div class="set_wrapp set_block">
            <div class="title"><?=GetMessage("COLLECTION_TITLE")?></div>
                <ul>
                    <?while($ar_fields = $res->GetNext()):?>
                        <?
                        $img_path = CFile::GetPath($ar_fields["PREVIEW_PICTURE"]);
                                <li class="item">
                                    <div class="item_inner">
                                        <div class="image">
                                            <a href="<?=$ar_fields['DETAIL_PAGE_URL']?>">
                                                <img src="<?=$img_path?>" alt="<?=$ar_fields['NAME']?>" title="<?=$ar_fields['NAME']?>">
                                            </a>
                                        </div>
                                        <div class="item_info">
                                            <div class="item-title">
                                                <a href="<?=$ar_fields['DETAIL_PAGE_URL']?>">
                                                    <span><?=$ar_fields['NAME']?></span>
                                                </a>
                                            </div>
                                        </div>
                                    </div>
                                </li>
                    <?endwhile?>
                </ul>
            </div>
        </div>