Hi there,
To make sure that CloudFlare is not the problem cause kindly detach your DNS servers from CloudFlare and disable the WP Rocket and CloudFlare plugins.
This will completely shit off the CloudFlare. Then you can see if it is the cause.
Also, I suggest that you make sure that you have correct file and folder permissions. For more information:
https://codex.wordpress.org/Changing_File_Permissions
Another point is that you get the Error 500 which is the internal server error. Kindly contact your hosting service provider and ask them to give you the latest error log. There you will get more detailed information of what is the problem cause.
Thank you.