If you want to reset a WordPress password using the Theme My Login plugin and are redirected to the Invalid Key error page, the following change to the resetpass-form.php plugin file (located either in /theme-my-login/templates/ or in the theme folder) may resolve the issue:
Replace these two field declarations
<input type="hidden" name="key" value="<?php $template->the_posted_value( 'key' ); ?>" />
<input type="hidden" name="login" id="user_login" value="<?php $template->the_posted_value( 'login' ); ?>" />
with the following:
<input type="hidden" name="key" value="<?php echo esc_attr($_GET['key']) ?>" />
<input type="hidden" name="login" id="user_login" value="<?php echo esc_attr($_GET['login']); ?>" />
Here is the thread that saved our web design agency a lot of work:
http://wordpress.org/support/topic/plugin-theme-my-login-fix-for-invalid-key-password-reset
