From My hosting support…
"the previous agent disabled all plugins, then re-enabled them, and it just started working again.
I also deactivated all 12 plugins you had active and left them off to test. I noticed Posts instantly were able to be added again, meaning it’s one of the plugins . So I happened to then deactivate the “Cornerstone” plugin, and it worked. When that is enabled, specifically, the Posts error comes up. I tested this several times, and it appears it’s that plugin specifically when enabled, as I left the other 11 on and it still allowed me to make new posts.
Please advise on a fix.