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: something might have gone wrong with the editing of the wp-config.php file. In that case, you could try to insert this line manually.