FTP to Zip takes browser based FTP backup of WordPress plus other folders. Proper error messages are shown if there is any defect in the installation.
Your Feedback is valuable. This plugin will work even if you have lost access to login to WordPress or it is unsafe to login (in case your site is under attack).
At any issues, please check the plugin's official support page at :
FTP To Zip official webpage.
Screen Shot Guided Tutorial :
How to use FTP To Zip Support is free as well. Simplify Backup of FTP Content. FTP to Zip is available in GitHub and Google Projects as WordPress FTP Full Backup, if you are a developer and want it for usage for other CMS or want to add features, simply download and proceed.