Solved (Publishing failed. The response is not a valid JSON response.)

Just q quick note on ye old

Publishing failed. The response is not a valid JSON response.

bug which plagues so many people. There are loads of posts, here and elsewhere, which list half a dozen potential culprits for people to check. As with too many other people, none of those worked for me. But, a very old modification did, somehow, magically work. No promises; it’s just what’s disappeared a day-long headache for me, for now!

I edited /etc/apache2/apache2.conf, to set

    <Directory /var/www/>
            AllowOverride FileInfo

instead of its default ‘AllowOverride None’. Then I restarted apache, via ‘apache2ctl restart’. Then, I could create a new page, _and_ visit it in a browser.
QED.

Context: WordPress 6.4.1 (ie the latest). Installed from the tarball (actually zip), on Linux Debian 11. PHP 7.4, mysql (actually, mariadb) 10.5.21, Permalinks set to Post Name. No activated plugins; none was added to the base installation (well, when things failed, I tried Really Simple SSL, to no avail, then deactivated it). The SSL certificate was successfully tested at SSL Labs. HTTPs used for visits/browsing as well as for identifying the site to WordPress. The Block Editor was used. Apache’s rewrite.load module enabled. The apache2 version seems to be 2.4.56. Browser cache frequently cleared.

Note that one of the proposed fixes was to use the Classic Editor. While this eliminated the error message for me, and allowed creation of new pages, I could not then visit those pages! The result was a ‘404 Not found’.