Problems with PHPLib Default Auth

Hi I set up a test page as described in the manual using Defualt Auth. When I load the page again with ?again=yes appended to the URL it displays the loginform, but when I submit these details I keep getting invalid username or password. I've set up other pages using Auth and Perms and these pages work fine. Has anyone else come across this. Any suggestions would be gratefully received. Thanks M@