in SSH/SFTP - "..." settings
Check "Server host key type" - EC
Under DH/KEX algorithms - "Add"
ECDH sha2 nisp521
ECDH sha2 nisp384
Or prioritize the list so those are the first two algorithms. The JSCH library has issues with DH group 16 512 exchange which it prefers to use in Core FTP Server build 780 (and possibly other builds)