WebBrowser login fails--sort of--on 1st attempt !

[Server Girl]

I am using a WebBrowser control, and I login using this code:

byte[] postArgs = System.Text.Encoding.ASCII.GetBytes ( LOGIN_PARAMETERS );

mainWebBrowser.Navigate ( LOGIN_URI, null, postArgs, "" );

mainControl.SelectTab ( browserTab );

mainControl.Focus ();

It appears to login successfully, but any link I click on in the logged in
page generates "Your validation has expired." and a demand to login.

Now, this only happens the first time I try to login. All subsequent
attempts work as expected.

OTH, if manually go to the browser tab, access the main page of that site
first, then login as above, all works perfectly from there.

Could it have to do with URL rewriting and cookies?

Thx, Jess