WHY? All settings are CORRECT!!!
With RapidWeaver these things happen at times. Just try again. Is this your first time publishing? If so then you may want to visit RW Preferences > Publishing and then choose something like 1 or 2 for maximum concurrent uploads.
Please note that RW is not as reliable as dedicated FTP software such as Transmit, Yummy, and other apps. I believe version 7 will get better with FTP, but versions 6 and earlier were “okay” but not hugely reliable.
If i got this error message i re-published all files agein and then it works. But this is not very often necessary.