There is a bizarre bug in MS Office that if any of the cell contains more than 911 characters, you'll have that error. I've had this problem many times and I have to develop an alternative writing the data one cell at a time using COM server.
I've verified this bug in Visual Basic, Visual C and reported this bug to MicroSoft long time ago but didn't really received follow up. You can find many others on the web too.
To find out if this is really the cause, you can run