Can you login to those accounts using the same key from the same local machine using any other SFTP/SSH client? E.g. PuTTY?
Btw, you should upgrade your WinSCP installation.
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m-jcb-Off-jcb, m-jcb-Off-
When trying to connect to the exact same sever with three different user name, only one succeeds to connect and the others just fail because "No supported authentication methods available".
Those two accounts have the same configuration and the same key set as the one that succeeds, and I don't see any difference.
Attached file shows the error codes when it fails to connect to the server.