If you can work with Windows registry, try deleting cached hostkey of the server from:
Also try prioritizing RSA at:
https://winscp.net/eng/docs/ui_login_kex
HKCU\Software\Martin Prikryl\WinSCP 2\SshHostKeys
Also try prioritizing RSA at:
https://winscp.net/eng/docs/ui_login_kex