I've called my webhost and there are no problems there.
Assuming Lightroom functioning normally (FTP working as intended, in general..), your FTP settings may not be correct, as entered / used by Lr/plugin.
What can I do to troubleshoot on the Lightroom end?
This script may help:
'FTP Test' - free, by me, available here:
If still no go, consider posting settings that work using 3rd party app, and settings as entered in Lightroom (omit password) - maybe somebody can spot something..
Also, give info: which proof site, how are you trying to upload to it..