Apologies in advance for seeming ignorant. (This is about my 100th WP install, but the first multisite)
a new, clean copy of WP 4.5.2, on a Windows 8.1 dev machine running XAMPP 1.8.3 /PHP 5.5.1
procedure (following the tutorial from this site):
installed WP as normal (on localhost - this is a development machine only)
added the define('WP_ALLOW_MULTISITE', true); line to wp_config -> Network setup menu item appears correctly. 'Install' button returns expected page with generated code.
Generated code added in place of the 'ALLOW_MULTISITE' line in wp_config as below:
Generated .htaccess lines added to the main site level .htaccess file (replacing old content)
At this point, any access to the site, or wp_admin subdirectory generate a 500 internal server error, and replacing the wp_config and/or .htaccess files w/backups has no effect.
This process was repeated 4 times, on 2 different development servers.
I must ask for help to find out what I have screwed up here.
Thanks in advance