I’m having the same issue with all of my WP sites hosted with 1and1.
I was able to get logged in for now by doing two things:
Copy then rename the wp-login.php file to something like adminlogin.php
Then edit the new file (adminlogin.php) on about line 841 where the form does a post to wp-login.php and change it to: <form name="loginform" id="loginform" action="<?php echo esc_url( site_url( 'adminlogin.php', 'login_post' ) ); ?>" method="post">
Hope that helps till 1and1 gets their ish together.