I recently switched from the regular Lubuntu LXDE/openbox to the i3 window manager under Lubuntu 16.04.
In either environment I can ssh with authentication forwarding activated into another machine via a vpn tunnel using:
ssh -A user@remote-machineA
On my local machine I have a ssh key for a git repository on remote-machineB. I can access the repository fine while on remote-machineA with LXDE but get a "Permission denied" using i3 to start with. Assuming my i3 does not forward keys like LXDE does. But I don't know what the missing piece is.
But unsure if this is the same problem as I experience, I tried the solutions proposed there but to no avail.