Maybe your server has a connection limit. You need to check with your server administrator. We cannot advice, why your server is failing authentication.

We are getting this issue sporadically since last month. for parallel file transferring ,we got this issue for couple of files but not for other files which transferred during same time.

Please find below error in details
TransferFile process Exception : WinSCP.SessionRemoteException: Server sent disconnect message
type 11 (by application):
"User Disconnected" ---> WinSCP.SessionRemoteException: Authentication log (see session log for details):
Using username "*******".
Authenticating with public key "rsa-key-****".
Authentication failed.

Can you please check and let me know on this?