I installed Zend server on my local machine. I found phpMyAdmin on my installation directory and I copied it into htdocs.
While running http://localhost/phpmyadmin it appears login page and when I enter username and password it does not redirect to the main page.
Instead it gives internal server error
Get the lastest version from zend website then You should try reinstalling your Zend Server CE, The correct phpmyadmin url is http://localhost/phpMyAdmin/ over windows machine otherwise you would get page not found.
Be sure that you don't have another apache instances running also in services panel.