When using Claim based authentication with SharePoint 2010, If you try export to excel functionality on any sharepoint list, it works fine with Office (Excel) 2010 client but with Office (Excel) 2007 client, it throws “you do not have adequate permissions to modify this list. Changes to you data cannot be saved” error message.

Office 2007 does not support FBA/Claim based authentication and directly tries to log-in using your windows credentials.

Resolution: (Work-around)
1) Log into the SharePoint site using forms authentication from office 2007 client
2) Open any existing excel document from the site. If you do not have an excel file on the server, upload an excel sheet to any of the document library.
3) It will be opened in browser.
4) Use Open in excel link on the page to open it in excel.
5) open it in read only or edit mode
6) Provide credential on the log- in popup
7) Now go to the SharePoint list and try exporting the list to excel option. It will work.