there might be an empty file in the startup folder. Default startup folder is: C:\Users\(User name\AppData\Roaming\Microsoft\Excel\XLSTART.
There might be also specific startup folders defined: Goto File menu | Options | Advanced. At the very bottom, there is a setting "At startup, open all files in". Check, if there is a folder entered, and if so, check if there are any files in this folder.