@Strahd Can you please get the output of the line:
grep -P 'Subsystem.*sftp' /etc/ssh/sshd_config
It should output something like:
Subsystem sftp internal-sftpIf it does not look like the above, edit the file (as root) look for the matched line and make it look like the above, then restart sshd service (systemctl restart sshd)
Thank you,