SFTP Interoperability failure with server using LibSSH 0.11

[MaxumJohn]

I am John O'Fallon, with Maxum Development, the maker of an FTP/HTTP/SFTP server called Rumpus FTP.

Describe the bug
Our SFTP server Rumpus was recently updated to use LibSSH 0.11, from previous releases built on LibSSH 0.10.6 and 0.9.8. When the server is built on LibSSH 0.11.0 or 0.11.1, Cyberduck can't connect, resulting in an error "Interoperability failure - SFTP - Underflow".

To Reproduce
Steps to reproduce the behavior:

1. Go to 'www.maxum.com' and click "Download" to download Rumpus for macOS. Rumpus for windows is currently built on LibSSH 0.10.6, and so it does not exhibit the problem (Cyberduck is compatible with this version).
2. Install Rumpus FTP for macOS, then enable the SFTP service on the SFTP Settings window.
3. Using Cyberduck, attempt to connect via SFTP to your test server.
4. See error

Expected behavior
Cyberduck should connect to the server normally. Again, if the server is built on LibSSH 0.10.6 or earlier, Cyberduck connects without error.

Screenshots

Desktop (please complete the following information):

• OS: macOS, Windows

Log Files
Here is a log file that includes the underflow error. It appears to log the error starting on line 1214.

cyberduck Underflow Error.log