[SOLVED] Quantity not updating in Shopping Cart

I am running 1.5.5.1

In the Shopping Cart, if I change the Quantity of a product that is in the Cart and click the Update icon, the page refreshes but the Quantity reverts to the original value.

I understand that the value is sent using HTTP Post and I think the redirects in my .htaccess file could be stripping these out, but I an unsure how to resolve the issue.

I tried removing the R=301 redirect from the RewriteRule...

From: RewriteRule (.*) http://www.MyDomain.com/$1 [L,R=301]

To: RewriteRule (.*) http://www.MyDomain.com/$1 [L]

..but it hasn't made any difference.

Can anybody suggest a solution?

Here is my .htaccess file:

IndexIgnore *.php
RewriteEngine On
RewriteCond %{HTTP_HOST} ^MySite\.com [NC]
RewriteRule (.*) http://www.MySite.com/$1 [L]
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L]
Options +FollowSymlinks
# Prevent Directory listing
Options -Indexes
# Prevent Direct Access to files
<FilesMatch "\.(tpl|ini|log)">
Order deny,allow
Deny from all
</FilesMatch>
# SEO URL Settings
RewriteEngine On
RewriteBase /
RewriteRule ^sitemap.xml$ index.php?route=feed/google_sitemap [L]
RewriteRule ^googlebase.xml$ index.php?route=feed/google_base [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !.*\.(ico|gif|jpg|jpeg|png|js|css)
RewriteRule ^([^?]*) index.php?_route_=$1 [L,QSA]

Thanks!

OK, I got it working

# Ignore Post Requests
RewriteCond %{REQUEST_METHOD} POST
RewriteRule ^ - [L]