qestion about ReturnUrl=%2f

Hi, all,

Recently I migrated a website from one sever to another and merely changed in the web config file from Version=3.5.0.0 to Version=4.0.0.0(also update on the new server IIS: the application pool to 4.0 from 2.0), but now every time I load the site URL, for example http://mysite.com, it appendsfunny ReturnUrl=%2f to it, likehttp://mysite.com/Login.aspx?ReturnUrl=%2f

and after I login, it still stays in the login page as the link is http://mysite.com/Login.aspx

and after another login, it will redirect me to the main page....

how can I remove ?ReturnUrl=%2f from the URL in the first place? (note: if I just typehttp://mysite.com/Login.aspx in URL bar, and hit enter, there is no isse)

thanks