Problem resolved...as curiously as it first came about...
It seems a permanent redirection code I had put in my .htaccess file and which had been working without issue for some time was causing the error.
The code isRewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /([^/]+/)*index\.php\ HTTP/
RewriteRule ^(([^/]+/)*)index\.php$ http://www.some-domain.com/$1 [R=301,L]
I repeat this code had been working flawlessly until ??? I don't know when exactly...software wise, the only major change was the upgrade to J1.5.23 from .22 version...
For more info you can look at the other thread where this code was first proposed...
forum.joomla.org/viewtopic.php?p=2490169#p2490169