Thanks for writing in.
There 3 reason why the error is showing.
- X theme cornerstone not the updated version
- Plugin conflict.
- Exceeding PHP memory limit, resulting in PHP memory exhaust.
- Wordpress core files corrupted.
First check if your setup is updated,
To update: https://theme.co/apex/forum/t/setup-updating-your-themes-and-plugins/62
Latest version guide: https://theme.co/apex/forum/t/troubleshooting-version-compatibility/195
Then check for plugin conflict.
You can do this by deactivating all third party plugins, and seeing if the problem remains. If it's fixed, you'll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
Then if still you could not be able to identify the problem.,
You can conctact hosting to get the error logs of the internal error and you can ask them to increase your memory php allocation
Error logs could help you identify which are the problem in your setup if you could not be able to identify with plugin conflict testing.
Let us know how it goes.