I think the URL tag should be: URL(c='default', f='user', args=['logout']) It's not really clear to me what you are trying to do. I would strongly suggest just using Web2py's built-in register/login/logout functionality.
If you are just starting out, I would advise creating a new application in admin and starting fresh. At least use web2py's user functionality in the beginning and then if you really want to change it later you can. But it works pretty well. And then you can focus on your application which is hard enough without re-creating the user functions.
