If you are using the Wordfance plugin, it is likely causing this issue.
Go to the Wordfance firewall settings, activate learning mode, open the page with this problem, update it, and activate Protection again. These steps are also described here:
https://wordpress.org/support/topic/admin-ajax-php-file-blocked/#post-13508663Thanks to that, Wordfance will learn which actions are safe and stop blocking page updates.