Editor will not open (WP 6.0.2, Gutenberg)

Hello,
in the WP dashboard, when I click on “Appearance/Editor” the browser will show only a blank white page. Source code shows plenty of HTML (looks like a normal page) but the browser shows nothing at all.

My configuration:

WordPress 6.0.2
Theme: twentytwentytwo
Gutenberg plugin installed and active
all in latest version
Server runs with PHP 8.0

The WP-Dashboard is accessible and seems to work fine. I can open pages; however, when I try to publish a page or update after changes, I get the error message
“Publishing failed. The response is not a valid JSON response”.
What is more, the preview pictures for embedded YouTube videos are missing in the editable view of my pages.
Strange: in spite of the error message, WP did save a test page I created, it is visible in Dashboard/pages.

When accessing the site as a user (i. e. without being logged in as administrator) all published pages are visible in all browsers.

Behaviour is the same in Chrome, Edge, Firefox and Vivaldi on PC – all browsers in current versions. Emptied browsers’ caches – but still no success.

I set up a new website two days ago. Edited successfully with blocks (tried different combinations, added and deleted content elements a lot, as I am not a professional); installed and deleted some plugins, which I suspect is the root cause of the problem. The editor worked absolutely fine – until today it didn’t anymore.

I tried to disable all plugins, but the editor page remains blank and white.
I found a thread in this forum, suggesting to change WordPress Address and Site Address under General Settings to https://www.domain.tld (adding the “www”), tried that, but it didn’t help.
I tried plugin Health Check & Troubleshooting which supposedly emptied the WP cache but it didn’t help either.

Any suggestions for a solution? What else can I do?
Thanks a lot for any help!

 

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