DSS is most likely broken at this server. OpenSSH is unable to connect as well when using DSS ("ssh -oHostKeyAlgorithms=ssh-dss ftp.indexes.nasdaqomx.com" command) and fails with "invalid format".
However, in addition to this, there seems to be an incompatibility in its RSA signature format as well, most likely this one. To work around this, upgrade to at least Rebex SFTP 2016 R1 (or the latest release) and make sure to prefer RSA host key algorithm. You should then be able to connect to the SFTP server again.