Many of us have faced the "Invalid token" issue while working on any joomla website.
Joomla tokens were added into joomla as a security feature for forms. Whenever a form was submitted it was also submitted with the form which was checked and validated after submission. If this was missing or incorrect you will get the Invalid token issue.
It was mostly included in the login and registration forms. It has been also added to logout form in recent version of joomla (1.6+).
The reason behind is clear either the token is missing or token is incorrect.
Missing token may relate that it has not been added to the form so simply add the following code will fix the issue.
echo JHTML::_( 'form.token' );
Invalid token is also caused due to any kind of cache being enabled on the website.
One Solution is to make sure sure your Login module is set to "No Cache" . Another thing you might want to check is if your System Page Cache plugin is switched off.