We have this error also, so I tried running the report as I have Excel 2010 and the user has Excel 2003. Still got the same error. My theory is that SSRS 2005 is using routines that are derived from an old code case.
It works on a SSRS 2008 instance.
I would be interested from anyone else who knows if there is a update patch for SSRS 2005 as we are not ready to cutover to SSRS 2008.