I would bet you are right. The problem you’ll have with dealing with most hosting companies help will come when you mention something RapidWeaver or any other ftp applications that aren’t covered by their “help” section.
That’s why I suggested using FileZilla as FastHost has a detailed step by step guide. So when you contact them you’re in their suggested app. Most of these help desk folks have cheat sheets and scripts for particular apps.
FileZilla is a great open source ftp application, it’s my go to ftp app for Linux. It’s easy to install and setup (should only take a few minutes). For most Mac users it appears to be intimidating as the ftp log file is right in front of you. Most other Mac ftp apps hide the log file.
Once you’ve figured out what exactly was causing the error then you can go back to RapidWeaver for publishing. Just apply any changes you made to credentials, paths, etc to the RW publishing setting.