I found a problem with my queries and it seems to me the following.
When I import an Excel file from Excel 2007 then you have to convert it to Excel 2003. That's fine but if you import it then to the database
the drop-down box does not understand the format if you run a query using that drop-down.
I haven't noticed anything odd myself, but to be honest I don't really import from excel often enough to comment.
Something to try though is to remove as much formatting from the excel file as possible to see if that helps. There maybe some odd formatting issue in the specific file your working on causing you problems.