details

The WordPress blog is located in filesystem location /var/www/html/ (i.e. index.php is in there, as are all the wp-*.php files such as wp-settings.php).

The WordPress blog's General Settings for WordPress Address (URL) and Site Address (URL) are both set to https://example.com/blog.

When I set the Permalink Settings to Month and name (or anything other than Plain), the .htaccess file is automatically modified to contain the following mod_rewrite block, and the URL paths adjust accordingly, e.g. /blog/2019/03/hello-world/, yet the accessing the URLs results in an Internal Server Error.

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /blog/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog/index.php [L]
</IfModule>

# END WordPress

The log reveals the following:

Request exceeded the limit of 10 internal redirects due to probable configuration error.
AH00121: r->uri = /blog/index.php, referer: https://staging.example.com/blog/
AH00122: redirected from r->uri = /blog/index.php, referer: https://staging.example.com/blog/
... the above line repeated ...
AH00122: redirected from r->uri = /2019/03/10/hello-world/, referer: https://staging.example.com/blog/

a hint of success...

With the Permalink Settings set to Plain, the .htaccess file is essentially empty, and the links work, though they are of the undesirable form /?p=123:

# BEGIN WordPress

# END WordPress

When I set Custom Structure, to /index.php/%year%/%monthnum%/%postname%/, the blog links are updated to the format /blog/index.php/2019/03/hello-world/ and they work, but now the URL is ugly (the /index.php/).

So why doesn't it work with /%year%/%monthnum%/%postname%/ and the automatically set .htaccess directives?

gory details

It shouldn't matter, but for the record: this WordPress blog is hosted in a Virtual Machine instance (VM instance) of the Compute Engine of Google Cloud Platform.

Solution

Remove /blog from the last rule (RewriteBase stays as before).

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /blog/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress