Really Simple SSL will try to flush the rewrite rules, because the domain has changed from http to https. This will prevent your site from showing 404 errors after the switch.
It seems that in some cases, either due to memory issues or a conflicting flush rule, this error can pop up. It will go away on refresh as this action will only execute once, and your site is switched to https.
If the error prevents the activation from completing (which is the case if the activate SSL notice does not go away), you can use the force activation method to bypass the activation procedure, by adding this to your wp-config.php:
define(“RSSSL_FORCE_ACTIVATE”, true);
There are safeguards to prevent this from happening, but apparently there are still some configurations in which this can occur.
Let me know if this helps!
