Set analysis based on date stopped working, why?

[#1]

I am using the following Expression (built using this as a working example originally) to count all Enquiries that have an Opened Date in the last calendar month...

=COUNT({<OPENEDDATE = {">=$(=monthstart(addmonths(today(),-1)))<$(=monthstart(today()))"}>} ENQUIRY)

This was working perfectly up until recently when I returned to work on the chart and develop it further, but is now not working at all.

Any Ideas as to why this has suddenly refused to work? There is a lot of recent data in there that is within the two date ranges.

[#2]

During this period did any changes were made to the QVW or an upgrade happened? Also check the format of OPENEDDATE and return on set expressions.

Kiran.