If you just updated to WordPress 5 and get the error “Updating failed” when you try to save or when it automatically saved:
There may be a conflict with a plugin or perhaps the platform you’re using.
A WordPress forum moderator doesn’t seem to think it’s a problem:
There have only been two reports for this, and the other was an API issue. :/ I’m only less concerned because of the lack of reproducibility.
And that doesn’t help us.
Luckily, the WP has released a plugin to help users get back to the Classic Editor after upgrading to 5.0: Classic Editor
Add and activate that plugin and you should be good to go until they figure out the issue.