Unable to set redirects from DA menu

When a specific user tries to use 'Site Redirection' from the da menu he gets the following error:

An error has occurred

Details

Unable to get Lock on /home/$USER/domains/$DOMAIN/public_html/.htaccess

He's the only user with this problem on the server, the .htaccess file exists and is owned by this user. When the file does not exist the same error occurs. We've checked and double checked all ownerships and permissions.