The 404 error when login in from a plugin can be fixed by replacing in admin/admin_globals.php
<input type="hidden" name="redirect" value="<?php echo $site_sess->url(ROOT_PATH."admin/".$self_url); ?>">
With this:
<input type="hidden" name="redirect" value="<?php echo $site_sess->url(str_replace("../../", "../", ROOT_PATH)."admin/".$self_url); ?>">
As of need login "again and again", it depends on what do you mean by "again and again"
If it's like every 15 minutes or an hour, then it should be fixed if you login as admin at members login form (on each non-admin page) and select "remember me" checkbox.