The log looks normal enough. SSH-2.0-0.0 is the server but unfortunately doesn't include the name of the implementation. (Others respond with e.g. SSH-2.0-OpenSSH_5.4.)
It's probably not a firewall issue since it would fail much sooner if Opus was being blocked from communicating.
If other programs can read the directory and Opus is talking to the server OK then it is odd that it isn't working.
My guess is that there's something unusual about the way the server is returning the folder listing, which Opus isn't handling correctly.
Would there be any way we could get temporary access to the server, or a similar one that exhibits the same problem, to debug things? If so, please email the details to crashdumps@gpsoft.com.au (or, for added security, encrypt the details and mail them there, then send a private message here with the password for the archive). If not, we could try adding some extra debugging into the code which would let you generate a log of the raw data the server returns for the directory listing, which might let us work out what's going wrong.
One random thought: Try changing the Misc > Use UTF8 setting for the site to either on or off (instead of the intermediate state, which means it tries to work it out automatically). If it's getting the encoding wrong, that could explain why no listing is coming out.
After changing the setting for the site in the FTP Address Book, please fully exiting Opus (via File / Exit Directory Opus) and then restart it before testing things, to make sure a cached connection isn't being used with the old setting still in place.