Theme editor in multi-site – can’t edit PHP files

I know this has been raised in many threads in many places but I am at a dead end.
On a fresh install of WP, multisite enabled, I can’t edit PHP files in the theme editor, IF the theme is ACTIVE in one of the sites.
In my setup, there are only two themes: Barebones and TwentyTWenty.
If I activate a theme in a site, then try to edit in the Network Admin, I get:
Unable to communicate back with site to check for fatal errors, so the PHP change was reverted. You will need to upload your PHP file change by some other means, such as by using SFTP.
If I DEACTIVATE the theme in the site, then go back to the editor in Network Admin, the error goes away.
Here’s what I’ve tried:

Deactivate (+/- reactivate) all plugins (only four installed)
Check all file permissions on the /wp-content/themes folder (OK)
Health check on the wp site is 100% OK

Insert lines in file.php as per page 1 of:
https://ww.wp.xz.cn/support/topic/cant-edit-main-theme-php-files-after-upgrading-to-4-9/

Couldn’t see anything of use, or any pointers. Is this a bug maybe?

• This topic was modified 4 years, 11 months ago by grindlay.

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