After upgrading to 5.9 I get blank screens when trying to edit or add posts

[eTurboNews]

I upgraded eturbonews.com to 5.9 and now I can no longer add or edit posts.
It gives a white blank screen.

I tried various browsers.
I changed the theme.
I disabled plug ins

Nothing helps.

[pbiron]

Sorry you're having problems after the update (and welcome to Core Trac).

When you "changed the theme", did you change to one of the bundled themes or some other theme?

Did you try deactivating all plugins?

[eTurboNews]

I only changed for a few minutes to 20/20 to test.
I deactivated ALL plug ins.

I changed back to our theme.
The blank screen on adding or editing posts remain.
Thanks

[pbiron]

Thanx. Do you have a "development" or "staging" environment of the site were you can try to make a few other tests without affecting the production/live version of the site?

[eTurboNews]

MY staging site is 2 years old.
I can of course clone this one = it will take hours because of the size.
Perhaps the only way out is the restore from our earlier back up with the previous WP version and we will lose today's news articles.?

[sabernhardt]

You could also try using the ​Classic Editor plugin as a (temporary) fix, so you can create and edit content on the live site. And you may want the Classic Widgets plugin to edit those, too.

[eTurboNews]

The Classic editor works when you set it as a default editor for all users and do NOT allow users to switch between classic and block

I hope WordPress will find a permanent fix to use the blog editor again.

[ldigregorio]

I get white screen when using single site, core 5.9, classic editor and gutenberg latest and switch to Appearance > editor. No other plugins on site cause it's a new site. So there's no conflict

[ldigregorio]

Just one other thing. The first page published without issue in firefox and now the second page won't save in either firefox or chrome. The error in both is "Publishing failed. The response is not a valid JSON response."

[ldigregorio]

Good news...found the reason for the error message: "The error in both is "Publishing failed. The response is not a valid JSON response." ---it's a security issue. Got past this. Was unrelated to core5.9, but the first one (white screen when trying to access beta editor in single site with no plugins but one (classic widget) I think probably is core 5.9 issue