Ubuntu Systemd Suspend Issue – Asus UX303UA Rebooting Instead of Resuming

I installed ubuntu 15.10 (kernel 4.2.0-22-generic) on a brand new Asus UX303UA laptop. Almost everything works fine, but I have some trouble with suspending: When the laptop is plugged in (charging), suspend and resuming from suspend works just fine. Strangely, when it runs on battery, it does not properly resume from suspend, instead it just reboots. It does not make any difference whether I suspend from the menu, close the lid or run systemctl suspend from the command line.

Any suggestions on how to tackle down the issue are welcome. So far I could not find anything relevant in syslog etc.

Edit: Some answers to the comments:

• The duration of the suspend state seems to make no difference, I tired between 20 seconds and several hours… always to same result.
• If I suspend while to laptop is plugged-in, then unplug and then try resume, the problems is the same. Similarily, if I unplug, then suspend, then plugin in and then try to resume, the laptop will reboot. So it seems the the computer needs to be plugged-in during suspend AND resume.

EDIT2: Some more infos to my installation:

• Running ubuntu in EFI-mode
• no swap partition
• main partition is encrypted