"Are you sure that you're using the FTP username and password of the settings of your Google merchant account and not your Google merchant account credentials ?"
Yes, I'm sure.
"If you trigger the cron task manually (set a really low frequency to the google products plugin) by opening the cron URL of the configuration via your browser, do you get any error message from Google ? If so, what does it says ?"
No Error Message
"Likewise, if you download the xml file directly from the google products plugin and upload it manually on your google merchant account, does it work ?"
Manual upload of xml file worked fine.