Você pode ver qual modelo "sales_email_order_items" se refere ao pesquisar
app/design/frontend/base/default/layout/sales.xml
app/design/frontend/base/default/layout/bundle.xml
app/design/frontend/base/default/layout/downloadable.xml
Por app/design/frontend/base/default/layout/sales.xml
exemplo, você verá:
<sales_email_order_items>
<block type="sales/order_email_items" name="items" template="email/order/items.phtml">
<action method="addItemRender"><type>default</type><block>sales/order_email_items_order_default</block><template>email/order/items/order/default.phtml</template></action>
<action method="addItemRender"><type>grouped</type><block>sales/order_email_items_order_grouped</block><template>email/order/items/order/default.phtml</template></action>
<block type="sales/order_totals" name="order_totals" template="sales/order/totals.phtml">
<action method="setLabelProperties"><value>colspan="3" align="right" style="padding:3px 9px"</value></action>
<action method="setValueProperties"><value>align="right" style="padding:3px 9px"</value></action>
<block type="tax/sales_order_tax" name="tax" template="tax/order/tax.phtml">
<action method="setIsPlaneMode"><value>1</value></action>
</block>
</block>
</block>
<block type="core/text_list" name="additional.product.info" />
</sales_email_order_items>
Aqui você pode descobrir o caminho: template="email/order/items.phtml"
Sua estrutura de diretórios /app/design/our_theme/default/default/template/email/order/items.phtml
parece errada, especialmente a parte /app/design/our_theme/default/default/template...
- deveria estar /app/design/our_theme/default/template...
(um padrão a menos, eu acho).