I saved your XML output you provided to a file 'test.xml' and added in the following header (to make it double-clickable in Windows):
<?xml version="1.0" encoding="UTF-8"?>
And then double-clicked to open in Excel; I also receive the error you reported:
This file cannot be opened because of errors. Errors are listed in:
C:\<user-path>\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.MSO\xxxx.log
So, I navigated to the path above and opened up the 'xxxx.log' (the number changes each time you open a file), and see this error:
XML PARSE ERROR: Undefined namespace
Error occurs at or below this element stack:
The 'undefined namespace' indicated can be found in your original document; in the line below the 'Workbook' line (note: the namespace prefix in your original document 'ns1' has apparently been substituted for 'ss' by Excel).
Your XML document doesn't associate the prefix 'pega' with any namespace; hence the XML parser error.
However: I think this is probably a side-effect of another issue: it seems that the <pega:withPage> element should have been picked up and re-written by the server (that is: you are attempting to use server-side Pega 'JSP TAGS' here).
Before you launch this 'raw' XML in EXCEL, you will need to make sure you first render the 'pega' tags.
Did you miss out the 'Property-Set-XML' in your Activity by any chance ?