Support for Excel password protection

I have been sent a password protected .xls file for which I have the password, but when I try to open it in LibreOffice on a Mac (OSX 10.9.4), I get the message “Read-Error. The encryption method used in this document is not supported. Only Microsoft Office 97/2000 compatible password encryption is supported”. I can view it, but how can I open it?

First assumption: You opened the file while still being attached to an email. In this case the file opens for safety reasons as “Read Only”. Please first save the file and then open it.

If the file was not attached anymore to an email but saved, I assume that you can view the xls in LibO but you cannot make changes. I would try to save the open file as an ods-file and then work on it.