Excel cannot open this file

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

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

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).

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

- 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