Hello,
It sounds like you may be using demdex-s2s-clients as the hostname, this is incorrect, the host name should be 's3.amazonaws.com'.
You should have been provided with a directory such as 'demdex-s2s-clients/mycompanyname', it may have even been provided as 's3://demdex-s2s-clients/mycompanyname'
Here is an example of what the WinSCP setup would look like:

Then you will want to enter the directory using the Advanced Site Settings:
