Post editor broken, after update to 5.8, from 4.6

URL is https://jasoncollege24.com server is CentOS, with cPanel installed.

When editing an existing post, updating fails with the message “Update failed. You are probably offline.”

When creating a new post, publishing fails with a similar message.

In the developer console, there are repeated 403 errors for POST, and GET

This is confirmed with Firefox, and Edge, with default themes, and no plugins. Enabling debug does not fix this error.

Adding, and enabling the Classic Editor pluggin is a workaround, but not the solution.

My own research leads me to things that either have no effect at all, or just make the entire thing unusable.

I’m not entirely sure what to provide you here, because the console is just dumping error after error. Please ask for what you need. thanks.

The page I need help with: [log in to see the link]