[Plugin: WP Super Cache] Not displaying message, writing cache files or Supercache folder.

I’ve used WP Super Cache a lot and never had a problem with it. I upgraded a client to WP 2.5.1 after which the Super Cache no longer worked. No message at the bottom of the page source at all. No cache files written. I deleted the cache folders and installed the latest version, it created the cache folder but no supercache folder. Again not adding any message to the bottom of the page or writing any pages.

I created a subfolder on the same server, installed WP and Super Cache, and it worked just fine. Will not work at the root installation. I’ve tried everything I can think of.

htaccess is

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
AddDefaultCharset UTF-8
RewriteCond %{REQUEST_METHOD} !=POST
RewriteCond %{QUERY_STRING} !.*s=.*
RewriteCond %{QUERY_STRING} !.*attachment_id=.*
RewriteCond %{HTTP_COOKIE} !^.*(comment_author_|wordpress|wp-postpass_).*$
RewriteCond %{HTTP:Accept-Encoding} gzip
RewriteCond %{DOCUMENT_ROOT}/wp-content/cache/supercache/%{HTTP_HOST}/$1/index.html.gz -f
RewriteRule ^(.*) /wp-content/cache/supercache/%{HTTP_HOST}/$1/index.html.gz [L]

RewriteCond %{REQUEST_METHOD} !=POST
RewriteCond %{QUERY_STRING} !.*s=.*
RewriteCond %{QUERY_STRING} !.*attachment_id=.*
RewriteCond %{HTTP_COOKIE} !^.*(comment_author_|wordpress|wp-postpass_).*$
RewriteCond %{DOCUMENT_ROOT}/wp-content/cache/supercache/%{HTTP_HOST}/$1/index.html -f
RewriteRule ^(.*) /wp-content/cache/supercache/%{HTTP_HOST}/$1/index.html [L]
</IfModule>

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress