A recent update to Apache (2.4.52) will cause URLs with spaces and other special characters to fail. For example:
Go to https://example.com/search
Search for "this and that"
This will take you to: https://example.com/search/node/this and that
and a 403 Forbidden message

Adding a "B" flag to rewrite rules in the .htaccess file in the top directory of Drupal 6 fixes this:
From:
RewriteRule ^(.*)$ index.php?q=$1 [L,QSA]
To:
RewriteRule ^(.*)$ index.php?q=$1 [B,L,QSA]

See: https://stackoverflow.com/questions/75684314/ah10411-error-managing-spaces-and-20-in-apache-mod-rewrite