"New-SFTPSession : The server response does not contain an SSH identification string."

In code that has been working perfectly for a while, New-SFTPSession has begun returning
"New-SFTPSession : The server response does not contain an SSH identification string."
I know that the target server has changed their certificate, but -AcceptKey ought to permit that. I can successfully connect using FileZilla and the SFTP protocol.
What might be going on?

#setup
$PWord = ConvertTo-SecureString -String $password -AsPlainText -Force #turn the password into a secure string
$Credential = New-Object -TypeName System.Management.Automation.PSCredential -ArgumentList $User, $PWord #use userid and secure string password to create credentials
$session =New-SFTPSession -AcceptKey -ComputerName $secureserver -Port $secureport -Credential $Credential #connect to FTP server

Start by reproducing the supplied New-SFTPSession command against the affected server, comparing the server's SSH identification response with the certificate change and the successful FileZilla connection. Trace New-SFTPSession's connection and host-key handling to determine whether the failure occurs before -AcceptKey is applied; done means identifying a reproducible cause and the appropriate fix or diagnostic guidance.