If you receive Microsoft Excel cannot access the file, There are several possible reasons error message you can use the following suggestions to resolve the issue.

If you are getting the Data source reference is not valid while trying to create a pivot table in Microsoft Excel, see this post for the fix.

Convert OpenOffice.org Calc spreadsheets into a Microsoft Excel file to take advantage of Excel's built-in tools. OpenOffice.org spreadsheets default to a .xsc file extension, which certain ...

If I trash the VBA Preferences file from ~/Library/Microsoft and reopen Excel, I can then open Solver again until the next time I open Solver Preferences, at which point VBA Preferences is recreated.