Thank you @martin,
It does indeed to appear to have been on JP Morgan's side. I had talked to them before, but they referred me to check with my sftp client support. Today, however, I tested it without making any changes and the connection went through on the 1st attempt in the gui with no prompt to update/add a host key or public key authentication error. I closed/reopened the connection 3 times in the gui to be sure and it worked every time. I set the host key that was used those times testing in my script and tested connecting via the script, and looking at the log, that succeeded as well. Thank you for your assistance. I am very glad to have this resolved.

hesske wrote:

I am also having an issue with connecting to JPMorgan. Via script and through the gui, I usually get: Remote side sent disconnect message type 2 (protocol error): "Connection blocked because server only allows public key authentication. Please contact your network administrator."
However, without making any changes, the connection sometimes completes(usually after clicking the Reconnect button several times). I've noticed too, mostly in the gui, that when I try to connect, at times I receive prompts to accept new host keys to the same server.
I am on Windows Server 2012 R2 running WinSCP v.5.17.6(Build 10516). I was having the same trouble on the version I was using before, 5.15. I am including an attachment of the log with some data scrubbed.

It looks like a server-side problem. Did you contact their administrator? Can you authenticate using any other SSH client?

I am also having an issue with connecting to JPMorgan. Via script and through the gui, I usually get: Remote side sent disconnect message type 2 (protocol error): "Connection blocked because server only allows public key authentication. Please contact your network administrator."
However, without making any changes, the connection sometimes completes(usually after clicking the Reconnect button several times). I've noticed too, mostly in the gui, that when I try to connect, at times I receive prompts to accept new host keys to the same server.
I am on Windows Server 2012 R2 running WinSCP v.5.17.6(Build 10516). I was having the same trouble on the version I was using before, 5.15. I am including an attachment of the log with some data scrubbed.

Your duplicate question on Super User:
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Hello,

I'm very new to WinSCP and configuring H-2-H keys. JP Morgan has updated their key on June 27 and now our WinSCP client seems to be failing to connect to their server to retrieve the files.

It seems very straightforward but I was unable to find anything in the WinSCP documentation, on JPM's site, as well as their Connectivity Guide.

How can I update my WinSCP system so that it is compatible with the new JPM key? I've tried PuTTYgen but JPM advised that I would not need to create any new keys myself. Furthermore, JPM didn't offer any help as WinSCP is a 3rd party tool... Not sure where to go from here and any assistance would be greatly appreciated.