specific text in .htaccess causes widget and New Post button issues

On my site, I’ve tracked down a block of text in .htaccess which seems to cause the problem with widget dragging and buttons not visible in teh New Post page. Removing this text fixes the problem, and replacing it brings back the problem.

For info, my plugins are: Akismet, BackWPUp, Better WP Security, Fast Secure Contact Form, Subscribe2 HTML
My theme is Twelve Plus Lite, which is a child theme of Twenty Twelve.
I previously tried disabling all plugins and switching to Twenty Eleven but this did not solve the problem.
I have not tried the manual reinstall.

# 5G:[QUERY STRINGS]
<ifModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{QUERY_STRING} (environ|localhost|mosconfig|scanner) [NC,OR]
RewriteCond %{QUERY_STRING} (menu|mod|path|tag)\=\.?/? [NC,OR]
RewriteCond %{QUERY_STRING} boot\.ini [NC,OR]
RewriteCond %{QUERY_STRING} echo.*kae [NC,OR]
RewriteCond %{QUERY_STRING} etc/passwd [NC,OR]
RewriteCond %{QUERY_STRING} \=\\%27$ [NC,OR]
RewriteCond %{QUERY_STRING} \=\\\’$ [NC,OR]
RewriteCond %{QUERY_STRING} \.\./ [NC,OR]
RewriteCond %{QUERY_STRING} \? [NC,OR]
RewriteCond %{QUERY_STRING} \: [NC,OR]
RewriteCond %{QUERY_STRING} \[ [NC,OR]
RewriteCond %{QUERY_STRING} \] [NC]
RewriteRule .* – [F]
</ifModule>

Enabling debug trace shows the following error message on the Widget screen. This is present whether or not the above text is in .htaccess

Notice: WP_Admin_Bar::add_node was called incorrectly. The menu ID should not be empty. Please see Debugging in WordPress for more information. (This message was added in version 3.3.) in /homepages/17/d406216176/htdocs/riderstest/wp-includes/functions.php on line 2944