It just happened again, this time I was just browsing the web, "modest" wasn't even set up. I noticed that a button border is missing in a dialog so I rebooted (using "reboot" from the command line) and it's screwed again. Interesting is that the battery charge level dropped dramatically at the same time, from 90% to 14%.

Tried changing the theme but that didn't help. I'll attach a screenshot to show you how it looks like:


I am real afraid of what's going on. Is that a faulty hardware? Is the filesystem getting corrupted somehow? The device is only two weeks old :-(