Why does scp -scp work but not scp without that flag

If I omit the destination file name and only specify directory, then it works:

scp /tmp/backup.tar.gz hostname:/home/user/

I don't understand why, but it might be solution to your problem as well.

You seem to be having two different problems:

• impossibility to login to shell (shell access can be disabled on the server) and
• impossibility for the library to open SFTP connection.

In the latter case you need to contact library vendor for assistance - there can be different reasons for the problem.

One thing you can do yourself is check that SFTP subsystem is really configured on the server. If SFTP server is not specified in config, some applications manage to open command channel and try to guess SFTP server location and run it via command channel. Libraries usually don't do this.