git remotes that have git-annex and git "installed" using the linux standalone tarball (or the OSX dmg) may not have git-receive-pack in PATH.
git annex sync and
git push etc work on such a remote,
it needs to have a dedicated ssh key that runs git-annex-shell and uses it
to run the command.
The assistant sets that up when it sets up a dedicated ssh key. However, if the server can be sshed into without a password using an existing ssh key, the assistant re-uses that key.
The assistant should either always set up a dedicated ssh key, even when not otherwise needed, or it should probe to see if git-receive-pack and git-send-pack can be run as part of its probing, and set up a dedicated ssh key when they cannot be. --Joey
After recent sshpassword changes, the webapp sets up a dedicated ssh key by default. If the user chooses to use an existing ssh key, it will be used. So this is less likely to be a problem. --Joey