403 Forbidden Error Page

Completely remove this new code from your root .htaccess file. There are a couple of separate issues/problems occuring on a relatively low number of Hosts so we are making a determination on how best to proceed. Most likely this code will just not be adaptable for some Hosts and cannot be used on those Hosts.

# BRUTE FORCE LOGIN PAGE PROTECTION
# Protects the Login page from SpamBots & Proxies
# that use Server Protocol HTTP/1.0 or a blank User Agent
RewriteCond %{REQUEST_URI} ^(/wp-login\.php|.*wp-login\.php.*)$
RewriteCond %{HTTP_USER_AGENT} ^$ [OR]
RewriteCond %{THE_REQUEST} HTTP/1\.0$ [OR]
RewriteCond %{SERVER_PROTOCOL} HTTP/1\.0$
RewriteRule ^(.*)$ - [F,L]

http://forum.ait-pro.com/forums/topic/403-error-after-upgrading-to-version-49-3/