To restart the portal:
1. Go to the admin centre > Portal actions > Restart this portal
Also, You can manually update the default studio with the following code.
<!-- Default studio template. Please do not modify -->
<div id="mainContent" class = "wrapper-body" role="main">
{% include 'Page Copy' %}
</div>
If it is not solved I would recommend you raise a support ticket with Microsoft.
Hope it helps.
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