[This is just a quickie, I’m putting it up here for anyone that received the messages above and searches for it online!]
Earlier today my team experienced some issues with a Reporting Services report that when attempting to export to Excel would cause Excel to throw the error “Excel found unreadable content” (I really should have got a screenshot but I didn’t think to do that. Sorry.)
Of course we went down the usual routes. Searching online threw up a Microsoft KB article Error message when you try to open a workbook in Excel 2007: "Excel found unreadable content in Book_Name" which describes a situation related to Analysis Services.
Turns out it was nothing at all to do with Analysis Services. The solution was actually a lot simpler – our Reporting Services server had run out of disk space and clearing down some space on that disk immediately solved the problem.
So, if you encounter the above error – check your disk usage!
Hope that helps!