As with 49kat, I been hit by the the dreaded Wake-From-Sleep issue on my Vox.
After many, many hours of testing and reconfiguration I have not been able to eliminate the problem.
Env: Latest patch installed. Never rooted. Ext SD card presence has no effect.
Set screen off to 15 sec.
Install any app.
Move that app to the SD Card.
Allow screen off and sleep. Wait at least 3 mins.
Wake from sleep.
Results: Shortly after waking I get an error stating the SD card was unexpectedly dismounted. A little grey triangle with an exclamation point in it appears at the top left corner of the screen. The Vox screen then becomes unresponsive. The Vox become unresponsive in under 60 sec. The On/Off button is unresponsive to the usual single press to turn on and off the Vox. Once the screen goes off the Vox appears completely dead and unresponsive.
Recovery: I can cause the Vox to 'cold restart' by the usual press and hold of the on off button. Once this state is reached I must always cold restart it.
Removing the 'apps' from the SD card does not eliminate wake from sleep death.
I have performed multiple factory restores (Vol+ and Start button). This does not result in returning the Vox to original (no sleep of death) behavior.
Do the initial configuration steps. Make no changes to the screen time outs or any other changes to the behavior. Do not add any apps, factory stock content. Let the Kobo reading app download all the content, new and old. Let the Vox sleep, wait 3 mins. No response from the screen, I sometimes have the 3 buttons at the bottom of the screen light up, but the screen stays black.
WiFi state is irrelevant.
My Vox is now unusable.
I really hope they figure out what's going on and fix it.