The new Integration FTP Agent (IG FTP) cannot connect to a SFTP server where the Server Host Key has been generated with RSA as host key algorithm.
The error in the Job Log is:
Connection attempt failed: Algorithm negotiation fail: algorithmName="server_host_key" jschProposal="rsa-sha2-512,rsa-sha2-256,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521" serverProposal="ssh-rsa,ssh-dss"
Component: Integration FTP Agent
Version: 21.0.9 and superior
By default, ssh-rsa and ssh-dss are not enabled in the Integration FTP Agent as they were considered as insecure in since versions Openssh version 8.8 and superior.
This is explained here:
In order to enable in the new Integration FTP (IG_FTP) agent the old insecure host and public key algorithms and ciphers, you will need to modify the startup command in the service manager configuration file to the following:
java -Djsch.server_host_key=ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,rsa-sha2-512,rsa-sha2-256,ssh-rsa,ssh-dss -Djsch.client_pubkey=ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,rsa-sha2-512,rsa-sha2-256,ssh-rsa,ssh-dss -Djsch.cipher=blowfish-cbc,3des-cbc,aes128-cbc,aes192-cbc,aes256-cbc,3des-ctr,aes128-ctr,aes192-ctr,aes256-ctc,arcfour,arcfour128,arcfour256 -jar ucxjftp.jar
See more information about the consideration of ssh-rsa as insecure in latest versions of Openssh here:
https://www.endpointdev.com/blog/2023/04/ssh-host-key/