Excel cannot open this file

[MacBoo_Who]

Version: 2008
Operating System: Mac OS X 10.4 (Tiger)
Processor: Intel

[All similar posts seem closed, so I'm creating a new one.]

ISSUE:
An .xlsx file (sent from our client) opens fine via Office 2007-Win, but Excel 2008-Mac claims "Excel cannot open this file" (despite a bar indicating 30% progress).

SPECS:
Macs running OS X 10.4.11, Office 2008 SP (12.1.1) only installed for users working on this one specific client.

ATTEMPTS:
- Office 2007-Win can save as .xls, but some content loses formatting (e.g. charts, etc.)
- Changing extension to .xml does not allow it to open
- Tried saving a new .xlsx copy to server (in case email attachment issues), but same result

 

[MacBoo_Who]

Forgot to point out that this file will be sent back-and-forth between the client and us, so any "convert-to-this/that-extension" work-arounds are not tenable solutions.

I also forgot to mention that Apple's Numbers.app WAS able to open the file (at 100% CPU). However, it listed ~33 errors re: conditional formatting, unsupported formulas, etc. Also, it can only export as .xls (23MB at that!) At least it shows me something (more than Excel can at this point)!