Comment 6 for bug 1605189

I also went into this issue "system is asking for authentication to suspend" running Xubuntu 17.04 Zesty Zapus.

But luckily I figured out, this behaviour came when I set "lock session automatically when screensaver is activated" in the security tab of XFCE power management. Setting it back to "never", suspending the system is working without authentication again.

Hope that hint helps finding some better solution.