Troubles on authenticating with pkexec

I'm using Ubuntu 14.10 dev with policykit-1 0.105-4ubuntu2 and on trying to change from gksu to pexec I'm ecountering some errors:

1. pkexec isn't showing a graphical frontend and asks directly in the terminal for the password.
2. After successfully authenticating pkexec closes mostly without giving an error message. Here is an example output:

sworddragon@ubuntu:~$ pkexec pcmanfm
==== AUTHENTICATING FOR org.freedesktop.policykit.exec ===
Authentication is needed to run `/usr/bin/pcmanfm' as the super user
Authenticating as: ,,, (sworddragon)
Password:
==== AUTHENTICATION COMPLETE ===
Hangup

But sometimes I'm getting also an error message:

sworddragon@ubuntu:~$ pkexec pcmanfm
==== AUTHENTICATING FOR org.freedesktop.policykit.exec ===
Authentication is needed to run `/usr/bin/pcmanfm' as the super user
Authenticating as: ,,, (sworddragon)
Password:

(process:1795): GLib-WARNING **: GChildWatchSource: Exit status of a child process was requested but ECHILD was received by waitpid(). Most likely the process is ignoring SIGCHLD, or some other thread is invoking waitpid() with a nonpositive first argument; either behavior can break applications that use g_child_watch_add()/g_spawn_sync() either directly or indirectly.
==== AUTHENTICATION COMPLETE ===
Hangup

Both times calling "echo $?" afterwards shows 129.