permalinks and foo.example.com

Just out of curiousity: did you try whether it works by adding /archives/ at the beginning of the line?

heh – out of curiosity I’ve just tested it quickly with the non mod-rewrite option – ie sticking index.php at the beginning of the path. With /archives/ and without both just delivered 404.

Well, a conspicious silence on this subject suggests that maybe I should be talking to my hosting service instead ??
FWIW, I’ve been troubleshooting. If this isn’t obvious to anyone else out there already, the problem seems to be the .htaccess file rather than whatever WP does when you switch on permalinks.
As soon as I insert the .htaccess file in /blog/, all urls inside /blog get resolved to
https://foo.example.com/cgi-bin/tester
as soon as the actual link is submitted to the server.
This happens whether or not I ‘update permalink structure’. The server is running OSX using Apache.
If I delete the content in .htaccess then everything is fine. So htaccess is obviously working, but the server is stumbling over the instructions.
BTW, the .htaccess code is:
Options FollowSymlinks
RewriteEngine On
RewriteBase /blog/
RewriteRule ^category/(.*)/(feed|rdf|rss|rss2|atom)/?$ /blog/wp-feed.php?category_name=$1&feed=$2 [QSA]
RewriteRule ^category/?(.*) /blog/index.php?category_name=$1 [QSA]
RewriteRule ^author/(.*)/(feed|rdf|rss|rss2|atom)/?$ /blog/wp-feed.php?author_name=$1&feed=$2 [QSA]
RewriteRule ^author/?(.*) /blog/index.php?author_name=$1 [QSA]
RewriteRule ^([0-9]{4})/?([0-9]{1,2})?/?([0-9]{1,2})?/?([_0-9a-z-]+)?/?([0-9]+)?/?$ /blog/index.php?year=$1&monthnum=$2&day=$3&name=$4&page=$5 [QSA]
RewriteRule ^([0-9]{4})/?([0-9]{1,2})/([0-9]{1,2})/([_0-9a-z-]+)/(feed|rdf|rss|rss2|atom)/?$ /blog/wp-feed.php?year=$1&monthnum=$2&day=$3&name=$4&feed=$5 [QSA]
RewriteRule ^([0-9]{4})/?([0-9]{1,2})/([0-9]{1,2})/([_0-9a-z-]+)/trackback/?$ /blog/wp-trackback.php?year=$1&monthnum=$2&day=$3&name=$4 [QSA]
RewriteRule ^feed/?([_0-9a-z-]+)?/?$ /blog/wp-feed.php?feed=$1 [QSA]
RewriteRule ^comments/feed/?([_0-9a-z-]+)?/?$ /blog/wp-feed.php?feed=$1&withcomments=1 [QSA]
And no, the ‘options FollowSymlinks’ makes no difference at this stage

Well, it was down to the way the hostiing service was configured. This from my host admin:

There’s a config directive called AllowOverride where you specify which config directives can be overridden by .htaccess. I originally had it set to allowing specific features which included ReWrite. However, I just set it to “All” and obviously, that seems to have done it. ??

So, it works now ??