You can see which template "sales_email_order_items" refers when looking into
app/design/frontend/base/default/layout/sales.xml
app/design/frontend/base/default/layout/bundle.xml
app/design/frontend/base/default/layout/downloadable.xml
In app/design/frontend/base/default/layout/sales.xml
for example you will see:
<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>
Here you can find out the path: template="email/order/items.phtml"
Your directory structure /app/design/our_theme/default/default/template/email/order/items.phtml
seems wrong, especially the part /app/design/our_theme/default/default/template...
- it should be /app/design/our_theme/default/template...
(one default less, i guess).