SFTP login accepted, SSH login denied by Synology NAS
Since this may help others... i had a similar issue, though with opposite behaviour as asked in this question: i could login via ssh, but not via sftp, resulting in errors like this one:
subsystem request failed on channel 0
It turned out the ‘application permission’ for the ‘FTP’ application (yes, also applicable to sftp) was disabled for my user. It can be enabled by editing the user account via the Synology DSM control panel.
Also make sure the SFTP service is actually enabled.