Theming login form just for specific user registration / Auto Assign Role Page On this page users gets automatically a role assigned by the Auto Assign Role module.

For a better user experience i added the login form also to this page. This works great:

In user--register.tpl.php I added the following code to print the login-form &lt;?php print(drupal_render(drupal_get_form('user_login_block'))); ?&gt;

But how do i THEME this login form on this specific page? So it doesn't affect the original form on /user/login?
You are rendering the user_login_block within your template, which is not rendered on the /user/login page.

As long as you have not placed a login-block, that you don't want to be affected, you should not have any problems.

You can use the template file block--user--login.tpl.php to theme this block or even use a form_alter hook to alter the form.