Whilst trying to add a robots.txt file to a server, I encountered a 403 Forbidden error. Having scoured the internet the only solution I found was to check whether certain plugins had modified the .htaccess file. Defender basically adds TXT as file extension. I've subsequently removed txt from that line and robots.txt now loads fine. Just thought I'd make you aware if anyone else has issues.
## WP Defender - Prevent information disclosure ##
Deny from all
## WP Defender - End ##