The document Panel was unable to load (Excel 2007)

We have a mix of Office 2007 and 2010 users, using sharepoint 2010.

Some particular Excel spreadsheets when opened from sharepoint using excel 2007, i am getting:

"The document Panel was unable to load. The document will continue to open. For more information contact your system administrator."

If you click ok to this message, you can keep on working on the spreadsheet.

However, with Excel 2010, there is no such issue.

Also, if i save the spreadsheet to the local drive, and open it locally with Excel 2007, i still get the same issue.

Anyone know what could be causing this? Is it just purely a compatibility thing between the versions of Excel?

Thanks

DM