After some trial and error I have found that 3.14.0-3ubuntu8 and subsequent versions of gnome-flashback has introduced an error where connections to a PPTP vpn fail with the following error:
NetworkManager[5971]: <error> [1428023479.735160] [vpn-manager/nm-vpn-connection.c:1778] get_secrets_cb(): Failed to request VPN secrets #2: (6) No agents were available for this request.
Downgrading to 3.14.0-3ubuntu7 resolves the issue.
I have reproduced this error by installing vivid into a virtualbox VM, and configuring a pptp vpn connection. It works if you use the default unity session, or the gnome one. Downgrading to gnome-flashback 3.14.0-3ubuntu7 resolves the issue.
I'm not going to bother attaching any further information unless asked, as I have narrowed it down to the specific update which has caused the regression and reproduced it on a fresh install.
Link to changelog for convenience: https://launchpad.net/ubuntu/+source/gnome-flashback/3.14.0-3ubuntu8
Enabling end-session-dialog we started using org.gnome.Shell dbus name. As expected this is causing problems - in this case nm-applet detects that shell is running and does not register agent. shell_version_
After short testing looks like this is fixed in upstream 3.16.0 version where I am exporting shell version, but it is set to 0. Needed callback is called, version is too old and nm-applet registers agent.
1. One way could be to patch network-
applet.c:44224 - if (!shell_debug) -> if (!shell_debug && !gnome_flashback)
2. Patch gnome-flashback to export ShellVersion or add network agent based on gnome-shell.