Actually I just tested and while I can now access cornerstone, it’s still not working.
This is the reply I had from host:
Hi James,
Thank you for your reply.
I am happy to inform you that the edit page is not showing the error 500 now.
There were couple of things causing the issue:
the memory limit was set in couple of places and was set to 2048MB which is a value the server ignores, because the max possible value for the PHP memory limit on the shared hosting is 512MB
the PHP version 7.2. That version appears not to be completely compatible with all features of your site and was causing issues. I have set PHP 5.6 to make the things work.
the max memory limit set in the wp-config.php file was set to 2048MB which is a value that the server ignores
Please feel free to contact us back if you need anything else.
After testing, although I can now access CS, this is what happens, when I try to save (see video) https://www.dropbox.com/s/85ewiqili7klv3s/save.mp4?dl=0