Symptoms:
SSH or WinSCP connection to ESXi host or vCenter Server Appliance fails a message similar to:
Couldn't agree a key exchange algorithm (available: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,)
The Couldn't agree a key exchange algorithm is expected behavior with older versions of PuTTy or WinSCP.
To resolve this issue, upgrade PuTTy or WinSCP to the latest version via the below links.
http://www.putty.org/.
https://winscp.net/eng/download.php
Try using PuTTy or WinSCP from a different computer/laptop if applicable.
If still receiving an error while trying to connect to vCenter after upgrading WinSCP, try from a different computer/laptop follow KB https://knowledge.broadcom.com/external/article?legacyId=2107727