I recently upgraded my site from Joomla 1.5 RC4 to Joomla 1.5 Stable. The process was fairly painless, essentially the same steps as upgrading any joomla release. Everything seemed to go smoothly on a copy of the site so I went ahead and upgraded the live site. Everything looked and ran great until I tried to login. After clicking the "Login" button I was redirected to a screen that said only "Invalid Token". I know joomla has a login form token generator and I figured it was the problem but I decided to do some google'n to see if it was a known problem. Sure enough, it is a known problem with a known solution.
"masterChief" (Andrew), at joomla.org has a solution here:
Here is the cause, Joomla 1.5 login forms need to have what is called a 'hash token' created that gets sent along to Joomla with login information of the form. From what I know, Joomla uses this technique to ensure that the login request is coming from itself and not an outside source like a bot or script attack. The code used to create this hash token changed from Joomla 1.0.x to Joomla 1.5. So, if you have a custom login module, or you are using an older login module from an RCx release then you need to update it.
If you do not know where to look but you are using the standard Joomla 1.5 login module. Either use FTP or SSH to edit 'modules/mod_login/tmpl/default.php' In that file look for the old code from above and replace it with the new code. If you are not familiar with either one of those methods you will have to hire someone to do it for you. If you would like assistance from MoeDesigns use the 'contact us' form to send us your details.
Once you have updated your module everything should work fine! If you are still having problems make sure that you have cookies enabled. Joomla can produce the same error if cookies are disabled in your browser.