WordPress depends on a variable, $_SERVER[“HTTPS”] to check if it is on SSL. As a consequence, when this variable is not set, and SSL is forced, WordPress tries to redirect back to http. This causes redirect loops on the back-end only, and not on the front-end.
Really Simple SSL fixes this by adding
$_SERVER["HTTPS"] = "on"
to your wp-config.php.
If you have activated Really Simple SSL, enabled SSL, and still have this issue, there might have gone something wrong with the editing of the wp-config.php file. In that case you could try to insert this manually.