I encountered an error today when I clicked on a Select button while setting up new entry in KPI list using Excel. A pop-up window shows up with SharePoint error "A web part on this page has property settings defined in tags that contain invalid xml".
The steps I took were:
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Open any KPI List
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Choose New -> Indicator from Excel Workbook
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On the "Sample Dashboard KPI Definitions: New Item" new page fill in valid values for the "Name" and "Workbook URL" text boxes.
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Click the "Select" image button to the right on the "Cell Address for Indicator Value" text box
After a quick investigation, I found that the button called a javascript function OpenExcel, and pass a number of parameters. One of the parameters are the relative location of _layouts/ExcelCellPicker.aspx.
Knowing this, I realise what is the problem. My site collection actually resides within managed path, e.g. http://myportal/sites/site1. However, I do not have a “/” level site collection, at http://myportal/. Thus, when the javascript is called, a pop-up window trying to access http://myportal/_layouts/ExcelCellPicker.aspx, which is invalid because I have not created a site collection yet.
In conclusion, when you ever encounter this error, just create a site collection at “/” level on your web application.
Yaohan Yapit 
Thank you! I haven’t found this information anywhere else. Neither my Site Collection admins or my SP Farm Admins were able to isolate the error until I sent them a link to this blog entry!
This is the exact problem we ran into. However, the ExcelCellPicker.aspx file resides in any sub-sites /_layout directory. If our sub-site is http://myportal/subsite/_layouts/ExcelCellPicker.aspx how do we get the KPI web part ot use that aspx page instead of the one at the site collection level?
Unfortunately, you cannot really change it without impacting overall SharePoint sites. Remember that /_layouts directory on the subsites are actually pointing to the same SharePoint hive directory. So as long as you have a site collection at the “/” level, it will just work.