Guttenberg editor - "Updating failed. The response is not a valid JSON response"

We are getting an error when saving posts in the Guttenberg editor - “Updating failed. The response is not a valid JSON response”. Here are no errors Clip2Net Menu and Yes: Clip2Net Menu

We tried the standard solutions - permalinks and the rest api ssl certificate, but the error still occurs. We turned off the plug-ins sequentially - it did not help. We changed the theme and hosting, and that’s when the save blocks in Guttenberg started working.

Found this thread WordPress: "Updating failed. Error message: The response is not a valid JSON response."

Question: maybe you have encountered such a problem and know the solution and it lies in the Cloudflare settings?

Thanks

May I ask have you checked for any requests being blocked or challenged under the Firewall tab → Overview?

Furthermore, is JSON (WP REST API / WP-JSON) enabled for your WordPress website or rather restricted, or disabled or blocked due to some security plugin?

I would suggest adding your hosting/server IP to Firewall → Tools → IP Access Rules with the action “allow” for your website.

Before moving to Cloudflare, was your Website working over HTTPS connection?
Is your WordPresss admin configured to work over SSL?

May I ask what SSL option have you got selected under the SSL/TLS tab at Cloudflare dashboard for your domain ( Flexible, Full, Full Strict … )?

May I suggest below article for troubleshooting:

Otherwise, you could try to enable the WordPress Debug mode and check for any clue, if so:

Nevertheless, despite of not knowing the WordPress version, have you tried using Classic Editor and Classic Widgets rather than the block editor?

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