After carefully checking the hosting settings again, I found out that Plesk’s firewall was too restrictive. Each API call was going through it and — in most cases — any non-text block was mistakenly detected as WordPress Unauthenticated Content Injection vulnerability, for some reason. I noticed it in the firewall’s log.
Whitelisting Gutenberg in the rules set and changing the firewall mode to “Detection only” fixed the issue.
Not sure if a Plesk’s update caused it. Nobody changed the firewall rules recently and the 403 issue appeared only a few days ago. I’ll report it to Plesk, in case it’s a bug.
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