You can try a different flavor of Excel output, or change the RSVP.FILE.EXTENSION.XLS setting:
1. Logon to Cognos Connection, Click toolbar, Click Server Administration.
2. Click the Services button
For both the BatchReportService and ReportService:
3. Click the Set Properties icon, click the Settings tab
4. Next to Environment, Advanced Settings, click Edit.
RSVP.CSV.DELIMITER ,
RSVP.CSV.ENCODING utf-8
RSVP.CSV.TERMINATOR CRLF
RSVP.FILE.EXTENSION.XLS TRUE