We need to know what client you’re using
Are you definitely choosing SFTP?
When you say can’t log in, what happens / what do you see?
same with WinSCP
It was happened after an update in
Update your Putty and WinSCP to the latest version from the authors website. (Not elsewhere)
Old versions of Putty/WinSCP use insecure keying methods which are no longer considered to be secure and are now blocked since a recent Debian update some time around November.