Hi I have had this issue since I updated about a month ago although admittedly i had not used the admin for a while before so not sure if that caused the issue or something else, I had nothing urgent to change so have kept coming back to it when I had time to go through the help, but now need to get it sorted.
The error appears when I try to edit any pages in cornerstone when I looked at the developer console errors the issue appears to be that in cornerstone there are 500 errors. All the live side of the site appears to be working fine. The URLS for the site and preview are the same.
I have tried turning off plug ins to see if that got me in, but it has not resolved it.
I have tried to follow your slef help topics on this, but none of the scenarios are what I am am seeing.