In version 18.104.22.168 the "pageSaveChecker.xml" file is no longer needed and does not exist anymore.
Some v 2.0 or v22.214.171.124 themes may still refer to the file and therefore the mashup pages with the
theme applied may come up blank or incomplete after upgrading to v126.96.36.199.
You may also see the following error: "Error 404: ProxyServlet: /mum/widget-catalog/pagesSaveChecker.xml
To fix the problem, just remove the following lines from your theme.html file and re-apply the theme.
< div class="mm_iWidget" id="pageSaveChecker">
< a class="mm_Definition" href="/mum/widget-catalog/pageSaveChecker.xml">< / a>