Excel files from my computer won't open - says a password issue

[TheBundo]

I don't use passwords on my Excel files, and nobody else that I email them to has an issue opening them on their computer. I do my invoicing with Excel, so this would come up all the time with customers if it were the case.

Yet, when I send a file to my Mogul, and try to open it, I get this message "Cannot open password protected Microsoft Excel workbooks". Any idea what I'm doing wrong?