Sorry if offended you with the answer, I apologize.
I just was testing on the Default Twenty Fifteen Theme, and as soon as I activated it I got a white Screen.
Please provide FTP access so that I could work this out, I believe it’s more that Just the Content Builder.
If you don’t like sharing an FTP access, please take the following action:
Connect Via FTP.
Navigate to themes/
Change the name of the directory /circleflip/ to something else, like /circleflip2/
Check the website.
If still white;
Please also change the name of the /twentyfifteen/
This should revert back to the default “another WP default theme” like Twenty Fourteen.
Please let me know and I’ll test your page with No plugins activated and one of the default themes running.
After that we should check the Memeory limit on the server.