I have encountered the same problem after a routine update of the x theme.
Either the backend worked or the cornerstone plugin did. I could not access backend at all when the plugin was active.
I changed the plugin folder name - deactivating it - and then removed and reinstalled the plugin from the X theme menu on the backend, which seems to have solved the problem. From what I can understand the theme’s update corrupted some file on cornerstone and then when loaded on backend it was crashing the backend.